Scanned-spot-array duv lithography system

ABSTRACT

A DUV scanned-spot-array lithography system comprises an array of phase-Fresnel microlenses, which focus multiple radiation beams through intermediate foci at the object surface of a projection system. The intermediate foci are imaged by the projection system onto corresponding focused-radiation spots on an image plane, and the spots expose a photosensitive layer proximate the image plane as the layer is scanned in synchronization with modulation of the beams. The modulators may comprise micromechanical shutters proximate the intermediate foci for ON/OFF switching, in series with transmission grating modulators for gray-level control, and the microlenses may also be actuated to provide dynamic beam centering control. A nodal line printing technique may be used to provide ultra-high-resolution and high-throughput maskless printing capability in conjunction with multi-patterning or dual-wavelength recording processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of thefollowing three applications, all of which name Kenneth C. Johnson asthe inventor:

-   -   U.S. Provisional Patent Application No. 61/857,166, filed Jul.        22, 2013 for “Scanned-Spot-Array DUV Lithography System”;    -   U.S. Provisional Patent Application No. 61/921,407, filed on        Dec. 28, 2013 for “Scanned-Spot-Array DUV Lithography System,        Addendum”; and    -   U.S. Provisional Patent Application No. 61/937,552, filed on        Feb. 9, 2014 for “Scanned-Spot-Array DUV Lithography System,        Second Addendum”.        (Patent and patent application numbers will be generally        abbreviated after the first citation by their three-digit        suffix, e.g., U.S. Provisional Patent Application No. 61/857,166        is referred to as the '166 application.)

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/801,919, filed Mar. 13, 2013 for “Scanned-Spot-Array EUVLithography System.” The '919 application claims the benefit under 35U.S.C. §119(e) of the following two applications, both of which nameKenneth C. Johnson as the inventor:

-   -   U.S. Provisional Patent Application No. 61/618,584, filed Mar.        30, 2012 for “Scanned-Spot-Array EUV Lithography System”; and    -   U.S. Provisional Patent Application No. 61/675,709, filed on        Jul. 25, 2012 for “Scanned-Spot-Array EUV Lithography System.”

This application incorporates by reference, for all purposes, the entiredisclosures (including any attached documents) of the above and thefollowing applications, all naming Kenneth C. Johnson as the inventor:

-   -   U.S. Pat. No. 6,498,685, filed Jan. 4, 2000 for “Maskless,        Microlens EUV Lithography System”;    -   U.S. Provisional Patent Application No. 60/116,074, filed Jan.        15, 1999 for “Spatially Modulated Microlens Array for EUV        Maskless Lithography” (incorporated by reference in the '685        patent);    -   U.S. Provisional Patent Application No. 60/124,140, filed Mar.        12, 1999 for “Improved Grating Modulator Design for EUV Maskless        Lithography” (incorporated by reference in the '685 patent);    -   U.S. patent application Ser. No. 13/103,874, filed May 9, 2011        for “Optical Systems and Methods for Absorbance Modulation”;    -   U.S. patent application Ser. No. 13/523,843, filed Jun. 14, 2012        for “Spot-Array Imaging System for Maskless Lithography and        Parallel Confocal Microscopy.”        This disclosure pertains primarily to DUV (deep ultraviolet)        embodiments of the '843 and '919 applications. Additional        prior-art references cited in the disclosure are listed in the        “References” section.

COMPUTER PROGRAM LISTING APPENDICES

The following text files were provided as “printed” (PDF) appendices tothe '166 application, and as computer-readable (txt) files, and areincorporated by reference herein. These files contain designspecifications for an illustrated embodiment of the invention, for usein the Zemax optical design program (Ref. 1). The content of these filesare described, and usage instructions are provided, in the writtenspecification below.

The PDF appendices are as follows

-   -   Appendix 1 titled “SiO2_266.ZTG” (1 page);    -   Appendix 2 titled “CaF2_266.ZTG” (1 page);    -   Appendix 3 titled “IF132_266.ZTG” (1 page);    -   Appendix 4 titled “SpotScanSystem_reverse.ZPL” (37 pages); and    -   Appendix 5 titled “SpotScanRMS.ZPL” (3 pages).

The computer-readable (txt) files are named as follows.

-   -   Text file 1 titled “SiO2_266_ZTG.txt” (30 lines);    -   Text file 2 titled “CaF2_266_ZTG.txt” (31 lines);    -   Text file 3 titled “IF132_266_ZTG.txt” (35 lines);    -   Text file 4 titled “SpotScanSystem_reverse_ZPL.txt” (2289        lines); and    -   Text file 5 titled “SpotScanRMS_ZPL.txt” (168 lines).        After downloading for use, the text file names need to be        modified by deleting the “.txt” extension and changing the last        underscore to a period (e.g., rename “SiO2_266_ZTG.txt” to        “SiO2_266.ZTG”). The files will be referred to by their modified        names hereafter.

BACKGROUND OF THE INVENTION

This application relates generally to scanned-spot-array lithographysystems, and more specifically to scanned-spot-array lithography systemsusing deep ultraviolet (DUV) sources.

Scanned-Spot-Array Optical Lithography is a maskless lithographicprinting method in which an array of diffraction-limitedfocused-radiation spots is raster-scanned over a printing surface (aphotosensitive optical recording medium) to synthesize a high-resolutionrecorded image. The spots may be individually modulated by a spatiallight modulator, or they may be collectively modulated by a singlemodulator.

A scanned-spot system described in U.S. Pat. No. 5,900,637 (the '637patent) comprises Fresnel zone plates 200, which convert parallel (i.e.,collimated) beamlets 212 of electromagnetic radiation into focusedbeamlets 213 converging to foci 215 on a printing substrate (the '637patent's FIG. 2; col. 2, line 55 to col. 3, line 8; and col. 4, lines4-27). The beamlets are individually modulated by micromechanicalshutters 219 between the zone plates and the substrate. Alternatively,the beamlets may be modulated by means of either shutters ormicromechanical mirrors preceding the zone plates in the parallel beampaths (the '637 patent's FIG. 3; col. 4, lines 23-44).

An alternative spot-scanning system disclosed in U.S. Pat. No. 6,133,986(the '986 patent) similarly uses an array 11 of light-modulatingelements such as micromirrors to modulate individual beamlets, which arefocused by a microlens array 2 onto foci on a printing surface 12. (Seethe '986 patent's FIG. 2 and col. 4, lines 28-48). In an improvementover the '637 patent the beamlets all pass through a common projectionaperture 7 of a projection system 1, which images the modulator elementsonto corresponding microlenses. (By contrast, the '637 patent's FIG. 3illustrates the beamlet light paths as being parallel andnon-intersecting in the space between the mirror array and the zoneplates.) The focusing elements in the '986 patent may becontinuous-profile microlenses, which have higher optical efficiency andless chromatic aberration than zone plates. Other possible microlensforms include micro-Fresnel lenses or binary optics (the '986 patent'scol. 13, lines 34-38). The '986 patent also describes methods forsensing and correcting positional errors between the microlenses and theprinting surface, e.g., by means of a piezoelectric transducer coupleddirectly to the microlens array (col. 19, line 19 to col. 25, line 9).

U.S. Pat. No. 6,897,941 (the '941 patent) discloses a spot-scanningsystem, which is similar to those of the '637 and '986 patents in thatit uses a spatial light modulator to modulate an array of paralleloptical beams, and focuses the modulated beams onto a spot array bymeans of microlens focusing elements. (See the '941 patent's col. 4,line 60 to col. 5, line 15 and col. 6, lines 24-40.) As illustrated inthe '941 patent's FIGS. 1 and 2, the modulated beams 106 are parallel inthe sense of being collimated between the collimating optics 103 andfocusing elements 114. The beamlets may be focused directly onto theprinting substrate 120 in the manner of the '637 and '986 patents'inventions, or the focused spots may be imaged through a demagnifyinglens 150 (col. 6, lines 53-55). Positioning errors may be controlled bymeans of a compensator system similar to the '986 patent's positioningfeedback and control mechanisms (the '941 patent's col. 3, lines 62-65and col. 11, line 66 to col. 12, line 21). In an improvement over the'637 and '986 patents, the system resolution is improved byincorporating a “beam shaper” (or “apodizer”) comprising an array ofshaped apertures in the parallel beam path (col. 2, lines 50-55; col. 3,lines 26-38; col. 5, lines 34-64). (the '986 patent discloses adifferent apodization technique in which the apodization is applied tothe projection aperture, not to an “array of shaped apertures”; see the'986 patent's col. 11, lines 15-20.)

SUMMARY OF THE INVENTION

U.S. patent application Ser. No. 13/523,843 (the '843 application)discloses design concepts for a scanned-spot system in whichfocused-radiation exposure spots are generated by imaging an array ofradiant-energy source spots through a projection lens onto a printingsurface at the projection lens's image plane. The source spots areformed in the projection lens's object surface by a microlens array.This is similar to the '941 patent's embodiment employing a“demagnifying lens” (i.e., a projection lens), but in an improvementover '941 the microlenses in the '843 application may be configured tocounterbalance and neutralize imperfect imaging characteristics of theprojection lens, enabling aberration-free point imaging over the entirespot array.

The microlenses can also (or alternatively) be configured to achievenarrow-band achromatization, intensity control, and polarization controlof the image-plane radiation. The exposure spots may be individuallymodulated by a spatial light modulator, or they may be collectivelymodulated by a single modulator (in which case the spots all printidentical patterns). If they are individually modulated, the modulatorelements may precede the microlens array as in the '986 and '941patents, or they may comprise high-speed micromechanical shuttersintegrated with an aperture array following the microlens array (see the'843 application's para. 0091 and FIG. 15).

U.S. patent application Ser. No. 13/801,919 (the '919 application)describes a specific embodiment of the '843 application's invention thatis adapted primarily for EUV (extreme ultraviolet) application, butwhich could also be used for DUV or visible-light lithography or otherforms of high-resolution printing. The design uses a Schwarzschildcatoptric projection lens comprising only two mirrors, and a novelmicrolens array comprising phase-Fresnel diffractive doublets in anachromatic Schupmann configuration. The Schupmann microlenses provideadvantages of high optical efficiency and low chromatic aberration, andthey can be formed on flat substrates (the '919 application's FIG. 10)using accurate microfabrication processes such as atomic layerdeposition and e-beam lithography (the '919 application's para. 0125).

The '919 application further discloses a modulation method in whichmodulator elements (indicated schematically as boxes 1105 in the '919application's FIG. 11) are located at the microlens foci (see the '919application's para. 0073). This differs from the '941 patent'sconfiguration employing a demagnifying lens, in which the modulation isapplied to parallel beams before they intercept and are focused by themicrolenses. It is similar to the shutter system illustrated in the '637patent's FIG. 2, except that in the '637 patent, the shutters cannot bepositioned close to the microlens foci because the foci are at theprinting surface. In the '919 application, the microlens foci are imagedthrough projection optics and onto the printing surface at reducedmagnification, so the microlenses can be comparatively large elements oflow numerical aperture and the modulators can intercept the beams at orclose to the intermediate foci. The modulators can consequently becomparatively small elements, as illustrated by element 1105 in the '919application's FIG. 11.

As noted in the '843 and '919 applications, and as used herein, the term“microlens” can generally denote a refractive and/or reflectivemicro-optic focusing device. For example, the micromirror illustrated inthe '919 application's FIG. 12 is a type of reflective microlens.Further, as used herein, the terms “lens” can generally denote arefractive and/or reflective focusing device.

The present application discloses embodiments of the '919 and '843applications' inventions that are configured primarily for DUVlithography. Embodiments of the invention provide a scanned-spot-arraylithography system and method in which multiple radiation beams arefocused through intermediate foci at the object surface of a projectionsystem, and the intermediate foci are imaged by the projection systemonto corresponding focused-radiation spots on an image plane. The spotsare scanned across a printing surface (i.e., a photosensitive layerproximate the image plane) in synchronization with modulation of theradiation beams to record a synthesized, high-resolution raster image onthe printing surface. The beam modulation is preferably effected bymeans of modulator elements such as micromechanical shutters proximatethe intermediate foci. (Element 1105 in the '919 application's FIG. 11schematically represents a modulator element in one embodiment of theinvention.)

In a first aspect of the invention, a scanned-spot-array lithographysystem comprises an array of microlenses and corresponding opticalmodulators, a projection system, and a scanning mechanism, wherein thearray of microlenses and corresponding optical modulators, theprojection system, and the scanning mechanism operate cooperatively toprint a lithographic image on a photosensitive layer when the layer ispositioned proximate an image plane.

In this system, each microlens receives radiation from a radiationsource and focuses it into a convergent beam converging toward acorresponding intermediate focus. Each convergent beam transmits throughand diverges from the corresponding intermediate focus, transmitsthrough the projection system, and is focused by the projection systemonto a corresponding focused-radiation spot on the image plane.

The optical modulator corresponding to each microlens is positioned tointercept the corresponding convergent beam proximate the intermediatefocus, and operates to modulate the radiation transmitting to thecorresponding focused-radiation spot. The scanning mechanismraster-scans the photosensitive layer relative to the focused-radiationspots in synchronization with the modulation to record a synthesized,high-resolution raster image on the photosensitive layer.

The system may further comprise collimation optics, which receivedivergent radiation from the radiation source and direct it intosubstantially collimated radiation intercepting the microlens array.

In various embodiments of the invention, the system according to thefirst aspect can be characterized by one or more of the followingattributes:

-   -   The microlenses are configured to substantially eliminate        geometric point-imaging optical aberrations at the        focused-radiation spots.    -   The microlenses are singlet microlens elements.    -   The microlenses are Schupmann doublets, each doublet comprising        first and second microlens elements. The first microlens element        of each doublet focuses radiation toward the corresponding        intermediate focus, the second element receives radiation        diverging from the intermediate focus and further diverges it;        and the first and second elements are configured to        substantially eliminate chromatic aberration at the        corresponding focused-radiation spot.    -   The microlenses comprise phase-Fresnel elements.    -   The projection system comprises at least one phase-Fresnel lens        surface.    -   Each modulator comprises a micromechanical shutter mechanism.    -   Each modulator comprises two proximate transmission diffraction        gratings, one of which is actuated to vary the convergent beam's        zero-order transmittance through both gratings between a        substantially zero-transmittance OFF state and a        high-transmittance ON state.    -   Each modulator comprises a micromechanical shutter mechanism for        effecting ON/OFF switching; and two proximate transmission        diffraction gratings, one of which is actuated to effect        gray-level control by continuously varying the convergent beam's        zero-order transmittance through both gratings between low,        high, and intermediate transmittance levels.    -   Each convergent beam traverses two microlens elements, one of        which is micromechanically actuated to provide spot centration        control.    -   Each convergent beam traverses two microlens elements, one of        which is micromechanically actuated to provide spot centration        control, and the microlenses are configured to substantially        eliminate geometric point-imaging optical aberrations at the        focused-radiation spots and to maintain substantial elimination        of aberrations over the full actuation range of the centration        control.

In a second aspect of the invention, the system according to the firstaspect is characterized by the radiation source being monochromatic, themicrolenses and the projection system being configured to producesubstantially zero-intensity nodal lines at some or all of thefocused-radiation spots, and the scanning mechanism raster-scanning thephotosensitive layer in the direction of the nodal lines.

In a third aspect of the invention, a method of printing a synthesized,high-resolution raster image on a photosensitive layer proximate animage plane uses a system according to the second aspect of theinvention. In this third aspect, the method comprises exposing thephotosensitive layer to a nodal line exposure pattern and a trimexposure pattern, wherein the system according to the second aspectperforms the nodal line exposure, and selected portions of the nodalline pattern are exposed by the trim exposure.

In some embodiments of this method, a scanned-spot-array lithographysystem according to the first aspect of the invention performs the trimexposure.

In a fourth aspect of the invention, the system according to the firstaspect is characterized by the radiation from the radiation sourcecomprising first and second distinct wavelengths, the microlenses andthe projection system being configured to produce intensity maxima inthe first wavelength coinciding with substantially zero-intensity nodallines in the second wavelength at some or all of the focused-radiationspots, and the scanning mechanism raster-scanning the photosensitivelayer in the direction of the nodal lines.

In a fifth aspect of the invention, a method of printing a synthesized,high-resolution raster image on a photosensitive layer proximate animage plane uses a system according to the fourth aspect of theinvention. In this fifth aspect, the method comprises exposing thephotosensitive layer to focused-radiation spots comprising intensitymaxima at a first wavelength coinciding with nodal lines at a secondwavelength, and the second wavelength inhibiting photo-activation of thephotosensitive layer by the first wavelength.

In a sixth aspect of the invention, multiple instances of the system ofthe first aspect are configured to operate in parallel and tosimultaneously print onto a photosensitive layer on a common imageplane, wherein the separate instances comprise separate microlensarrays, modulators, and projection systems.

In a seventh aspect of the invention, a method of printing asynthesized, high-resolution raster image on a photosensitive layerproximate an image plane, comprises directing radiation from a radiationsource through an array of microlenses and corresponding opticalmodulators, through a projection system, and onto the image plane in amanner so as to generate focused-radiation spots; and operating ascanning mechanism to raster-scan the photosensitive layer relative tothe focused-radiation spots in synchronization with the modulation torecord the synthesized, high-resolution raster image on thephotosensitive layer.

In this method, the focused-radiation spots are generated as follows.Each microlens receives radiation from the radiation source and focusesit into a convergent beam converging toward a corresponding intermediatefocus, each convergent beam transmits through and diverges from thecorresponding intermediate focus, transmits through the projectionsystem, and is focused by the projection system onto a correspondingfocused-radiation spot on the image plane, and the optical modulatorcorresponding to each microlens is positioned to intercept thecorresponding convergent beam proximate the intermediate focus, andoperates to modulate the radiation transmitting to the correspondingfocused-radiation spot.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings, which are intended to be exemplary andnot limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a cross-sectional view of the lithography systemoptics in a particular embodiment;

FIG. 2 is an expanded view of region 2 in FIG. 1B;

FIG. 3A illustrates a phase-Fresnel microlens in the meridionalcross-sectional plane of FIGS. 1A, 1B, and 2 (region 3A in FIG. 2) andFIG. 3B illustrates the microlens in plan view;

FIG. 4 is an enlarged view of region 4 in FIG. 3A, illustrating detailof the phase-Fresnel facet structure;

FIGS. 5 and 6 are enlarged views of corresponding regions 5 and 6 inFIG. 1B, illustrating the projection system's phase-Fresnel surfaces;

FIG. 7 schematically illustrates the printing process;

FIG. 8 tabulates design parameters and equations relating to systemprinting performance for the embodiment of FIGS. 1A and 1B;

FIG. 9 illustrates an array of 19 projection lenses covering a 300-mmsemiconductor wafer (in plan view);

FIG. 10 illustrates 37 projection lenses covering a 450-mm wafer;

FIG. 11 outlines the functional form of the microlens grating phase;

FIGS. 12A-12C conceptually illustrate a process for manufacturingphase-Fresnel grating structures;

FIG. 13 is an enlarged view of region 13 in FIG. 5 (with exaggeratedaspect ratio), illustrating a Fresnel facet structure that could be usedon the peripheral region of surface 108 in FIG. 1B;

FIG. 14 tabulates design data corresponding to FIG. 13;

FIG. 15 tabulates calculated optical performance data corresponding toFIG. 13;

FIG. 16 is an enlarged view of region 16 in FIG. 6 (with exaggeratedaspect ratio), showing an illustrative Fresnel facet design for theperipheral region of surface 109 in FIG. 1B;

FIG. 17 tabulates design data corresponding to FIG. 16;

FIG. 18 tabulates calculated optical performance data corresponding toFIG. 16;

FIGS. 19A-19D illustrate an optical modulator (in region 19A of FIG. 2)comprising paired apertures, which are actuated to move in oppositedirections;

FIG. 20 illustrates a backside wafer alignment technique;

FIGS. 21A and 21B comparatively illustrate a singlet microlens (FIG.21A) and a Schupmann doublet microlens (FIG. 21B);

FIGS. 22A-22D illustrate several variant microlens forms;

FIGS. 23A and 23B illustrate an optical modulator comprising proximatetransmission gratings;

FIGS. 24 and 25 illustrate a rectangular aperture geometry, andassociated equations, used for nodal line printing;

FIG. 26 illustrates an elongated focused-radiation spot resulting fromaperture narrowing;

FIG. 27 illustrates an array of 38 projection lenses, with narrowed,rectangular apertures, covering a 300-mm semiconductor wafer (in planview);

FIGS. 28A and 28B tabulate equations defining several alternative pupilfunctions and associated focal-plane field amplitudes;

FIG. 29 illustrates a rectangular-aperture pupil function, with andwithout apodization, and FIG. 30 illustrates the associated focal-planefield intensity;

FIG. 31 illustrates the pupil functions used for nodal line printing,and FIG. 32 illustrates the associated focal-plane field intensity;

FIGS. 33 and 34 illustrate two alternative lens forms for producing thenodal lines;

FIG. 35 illustrates a double-peak focus spot used for printing nodallines;

FIG. 36 illustrates alternative pupil functions used for printinghigher-density nodal lines using dipole illumination, and FIG. 37illustrates the associated focal-plane field intensity;

FIG. 38 tabulates equations related to an interleaved raster scanmethod;

FIGS. 39A-39C and 40-44 illustrate the interleaved raster scan method;

FIG. 45 illustrates an alternative scan configuration that makes moreefficient use of a circular image field;

FIG. 46 illustrates a laser beam scanner used for illumination strobingwhen a continuous laser source is used with the interleaved raster scanmethod;

FIGS. 47-52 illustrate alternative microlens aperture geometries andarray configurations;

FIGS. 53A and 53B tabulate equations for a 193-nm immersion (“193i”)design example;

FIG. 54 illustrates the microlens array aperture dimensions andclearances for the 193i design example;

FIG. 55 illustrates an optical modulator comprising a shutter mechanismfor ON/OFF control and a grating modulator for gray-level control;

FIG. 56 illustrates spot-generation optics including a movablemicrolens, which provides dynamic spot centering control;

FIG. 57 tabulates equations associated with spot centering control;

FIG. 58 illustrates a Schupmann-type microlens doublet with a movableelement for spot centering control; and

FIG. 59 schematically illustrates the components of a scanned-spot-arraylithography system and their functional relationships.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The Description of Specific Embodiments is divided into two parts: Part1 discloses a detailed optical design for a scanned-spot-arraylithography system that is configured to operate with a 266-nm diodelaser. The diode laser has a high repetition rate (e.g. 80 MHz), whichis advantageous for high-throughput maskless lithography. Part 2discloses an alternative “nodal line printing” method, which couldachieve higher throughput and better print resolution with thecomparatively low repetition rate (e.g., 6 kHz) of a 193-nm or 157-nmexcimer laser. With this approach, linear interference nulls in thefocused-radiation spots are used to print narrow line features. Denseline patterns can be formed by using prior-art multi-patterning andpitch division techniques, or by using a non-linear, dual-wavelengthrecording process. (Part 1 is based primarily on the '166 application,and Part 2 is based on the '407 and '552 applications.)

Part 1 1. Overview

An embodiment of the invention is adapted primarily for DUV application,using a dioptric projection lens similar to that depicted schematicallyas assembly 303 in the '843 application's FIG. 4, although the '843application did not disclose an actual optical design for the projectionlens. The '919 application provided design data for an EUV Schwarzschild(catoptric) projection lens, which could be used for DUV, but a dioptriclens can achieve better imaging resolution. The system employs microlensfocusing elements (301 in the '843 application's FIG. 4), which may beconfigured to compensate for the projection system's imperfect imagingcharacteristics, but the dioptric projection system reduces the amountof aberration correction required in the microlens design.

The projection lens is designed to operate in immersion mode, similar toprior-art 193-nm immersion lithography lenses (for semiconductor waferprocessing), but using a frequency-quadrupled diode laser at a 266-nmwavelength. The diode laser has a comparatively high repetition rate(e.g., 80 MHz versus 6 kHz for a 193-nm excimer laser), which may beadvantageous for high-throughput maskless lithography. The diode laser'slonger wavelength would limit print resolution, but this limitation canbe largely overcome by using high-refractive-index optical materials,which could be used at 266 nm even though they do not yet meetrequirements for 193-nm lithography. (Optical absorption, scatter, andbirefringence tend to be much lower at longer wavelengths.)

A limitation of the 266-nm diode laser is its comparatively widespectral bandwidth, about 50 pm (picometer), compared to less than 1 pmfor line-narrowed excimer lasers. To accommodate the spectral bandwidth,the projection system includes two phase-Fresnel, diffractive lenssurfaces as achromatizing elements. (The phase-Fresnel surfaces areannular-zone diffraction gratings blazed for maximum efficiency in the+1 or −1 diffraction order.) Two diffractive surfaces suffice to correctboth axial and lateral color. In addition, the diffractive surfacesprovide the functionality of strong aspheric elements. (The system hasno aspheric refracting surfaces.) The combination of phase-Fresnelprojection lens elements and aberration-correcting microlenses make itpossible to achieve design optical aberrations below the milliwave(0.001 wave) level across the entire image field and over the laser'sfull wavelength spectrum.

The microlens array (element 301 in the '843 application's FIG. 4) couldcomprise Schupmann diffractive doublets similar in function to thatillustrated in the '919 application's FIG. 10 to minimize chromaticaberration. But the microlenses' chromatic aberration over the 50 pmsource bandwidth would be insignificant (less than 1 milliwave) due totheir small focal lengths, so in the preferred embodiment themicrolenses are diffractive singlet elements. (Schupmann doublets couldhave advantages in alternative embodiments.) The microlenses arepreferably phase-Fresnel elements, similar to the projection lens'sdiffractive surfaces, which can be fabricated using techniques such asatomic layer deposition and e-beam lithography.

The projection lens images a 40-mm-diameter object field onto a1.6-mm-diameter image field at 25× reduction. The system iscomparatively small, with a maximum lens diameter of 52 mm (compared toabout 250 mm for prior-art immersion lithography systems, e.g., U.S.Pat. No. 8,355,201). Multiple such projection systems can operate inparallel on a single wafer to achieve printing throughput of ordertwenty 300-mm wafers per hour. This is significantly lower thanprior-art mask-projection scanners, which can process more than 200wafers per hour, but the laser power requirement is also commensuratelylower. The spot-scanning system's scan speed would also be comparativelylow, e.g., 35 mm/sec, in contrast to prior-art mask-projection scanners,which scan the wafer at about 700 mm/sec and the mask at 2.8 m/sec whilemaintaining nanometer-scale dynamic alignment (Ref. 2). Thespot-scanning system can use a backside alignment technique foraccurate, real-time positional feedback, enabling superior overlaycontrol for multi-patterning lithography.

The small-scale projection lens design has multiple advantages overprior-art, monolithic projection lenses. Small-scale lens elements useless glass volume per unit aperture area and can be manufactured moreeasily. Geometric and chromatic optical aberrations generally scale inproportion to the lens size. Thermally-induced aberrations are also lesssignificant with small-scale lenses, and smaller elements achievetemperature equilibration more quickly. The effects of opticalabsorption, refractive index inhomogeneity, stress birefringence,bubbles and inclusions in the lens glass all scale in proportion to thein-glass optical path length, which is reduced with small-scale lenses.Also, wafer non-flatness would have less impact over the comparativelysmall image field of a small-scale projection lens.

2. Illustrative Design Configuration

FIGS. 1A and 1B show a cross-sectional view of the lithography systemoptics in a particular embodiment. Laser radiation originating fromsource point 101 is collimated by collimator 102, is focused by amicrolens array through intermediate foci at the object plane of aprojection system 103, and is modulated by elements of a spatial lightmodulator (SLM) at the intermediate foci. The microlenses and modulatorsare disposed on opposite sides of a microlens/SLM plate 104. Theintermediate foci are imaged by the projection system onto a printingsurface 105, which is optically coupled to the projection lens throughan immersion fluid. The printing surface is raster-scanned insynchronization with the modulators to synthesize a high-resolutionprinted image. Two limit rays 106 at the edge of the image field areillustrated in FIGS. 1A and 1B.

The optics are configured to work with a 266-nm diode laser beingdeveloped by Coherent Inc. (Ref. 3), based on its Paladin (355-nm)product platform, with a power rating initially targeted at 1.5 W and arepetition rate of at least 80 MHz. The diode laser's comparatively widespectral bandwidth (50 pm) necessitates the use of an achromaticprojection lens. The wide bandwidth is a consequence of the laser's veryshort pulse duration, of order 15 picosecond (although this is not shortenough to induce significant optical nonlinearity in the projectionlenses, Ref. 4).

Referring to FIG. 1B, the collimator and projection system designs haverotational symmetry around an optical axis 107. The collimator 102comprises an achromatic air-spaced doublet with a biconvex calciumfluoride (CaF₂) element CL1 and a piano-concave synthetic fused silica(SiO₂) element CL2, both of which have spherical surfaces. Thetransmitted beam from the doublet is approximately collimated. Inprinciple, off-axis phase-Fresnel microlenses could be used toneutralize chromatic aberration in the collimator, but the microlensdesign is simplified by using a substantially achromatic collimator. Theaberration-correcting function of the microlenses obviates the need foraspheric elements in the collimator.

The planar surface of CL2 is in close proximity to the top surface ofplate 104. FIG. 2 is an expanded view of region 2 in FIG. 1B, showingthe edge portions of lens element CL2 and plate 104 intercepted by limitrays 106. The plate thickness is 1.56 mm, and a 10-micron air spaceseparates the plate and CL2. (In alternative embodiments CL2 could beoptically contacted to the plate.) The microlenses (e.g., microlens 201,shown in an expanded view of FIG. 2 region 3A in FIG. 3A) are formed onthe plate's top (CL2-facing) surface. The intermediate foci (e.g., focus202) are proximate the plate's bottom surface, which provides asubstrate for the SLM. (A modulator element is illustrated schematicallyas box 203 in FIG. 2.) The CL2 planar surface can serve as a substratefor a microstructure such as that illustrated in the '843 application'sFIG. 47, which controls polarization and intensity. (Alternatively, themicrolenses may be formed on the CL2 planar surface andpolarization/intensity control structures may be formed on top surfaceof plate 104.)

Referring again to FIG. 1B, the projection optics comprise 15 lenselements PL1 . . . PL15, which are all SiO₂ and all have spherical orplanar surfaces, except that surface 108 on PL1 and surface 109 on PL12comprise phase-Fresnel layers on spherical SiO₂ substrates. In addition,a form-birefringent polarization-control structure similar to thatillustrated in the '843 application's FIGS. 42 and 43 may be formed,e.g., on spherical surface 110 of PL11 or on a buried planar surface 111within element PL10. An aperture stop 112 may be located between PL11and PL12. The last lens element PL15 is piano-convex, with its planarside optically coupled to the printing surface 105 through a 0.5-mmlayer of immersion fluid (DuPont IF132, Ref. 5), enabling operation at anumerical aperture of 1.2. Alternatively, the printing surface may beoptically contacted to a solid, transparent cover plate, which isoptically coupled to the last lens element through an immersion fluid.(Insertion of an SiO₂ cover plate would have no effect on the opticaldesign other than to reduce the PL15 thickness by an amount equal to theplate thickness.)

FIG. 3A illustrates a phase-Fresnel microlens 201 (one of an array ofmicrolenses on plate 104) in the meridional cross-sectional plane ofFIGS. 1A, 1B and 2. (A “meridional” plane contains axis 107.) FIG. 3Bshows a plan view of one half of the microlens on one side of themeridional plane. (The microlens design has bilateral symmetry acrossthe meridional plane.) The scales in FIG. 3B are marked in micron units.The microlens clear aperture 301 has a diameter of approximately 100micron, and the microlens surface has four facet steps 302, 303, 304 and305. The microlens apertures and facet boundaries are not exactlycircular, and they vary slightly in shape across the microlens array,due to aberration in the collimation and projection optics and themicrolenses' aberration-correcting design form.

Region 4 in FIG. 3A is shown in an enlarged view in FIG. 4, illustratingdetail of the microlenses' phase-Fresnel facet structure for microlens201 in a preferred embodiment. The minimum facet width is approximately5 microns. The Fresnel facets could be etched directly into the SiO₂plate 104 as a linear-ramp profile (e.g., using gray-scale lithography),but they are preferably formed as a multilayer “staircase” pattern, asillustrated, comprising layers of a material of high refractive indexsuch as hafnium oxide (HfO₂, also referred to as hafnia) or siliconnitride (Si₃N₄). (A method for fabricating such structures will bedescribed in section 6.) A diffractive structure formed in a high-indexmedium will generally have a shallower grating profile, higher opticalefficiency, and less optical scatter than one formed directly in SiO₂.(The form-birefringent polarization-control structures are alsopreferably formed in a high-index medium such as HfO₂ or Si₃N₄, asdescribed in the '843 application.) Using HfO₂, the facet heights areapproximately one-quarter micron.

The projection system's phase-Fresnel surfaces 108 and 109 have a formsimilar to the microlenses, as illustrated by the enlarged views of FIG.1B regions 5 and 6 shown in corresponding FIGS. 5 and 6. (Phase-Fresnelsurface 108 has negative optical power, and surface 109 has positivepower.) A 1-micron dimensional reference is shown in the figures. Thefigures represent the diffractive surfaces near the periphery of thelens apertures where the grating period is smallest. The minimum periodis 2.16 micron for surface 108 and 1.04 micron for surface 109. Thephase-Fresnel surfaces have a form similar to the microlenses (e.g.,HfO₂ layers with a facet height of approximately one-quarter micron).

The following sections provide additional detail on the system'scomponents, design methodologies, and alternative embodiments.

3. Printing Process

FIG. 7 schematically illustrates the printing process, and FIG. 8tabulates design parameters and equations relating to system printingperformance. As illustrated in FIG. 7, the printing surface 105 (shownin plan view) scans in direction 701 so that each focused-radiation spot702 of a spot array traces a raster line 703 in synchronization withmodulation of the spot intensity. The spots are centered on a squaregrid distributed over a square exposure field 704, which is inscribedwithin a circular image field 705. (In alternative embodiments, thespots might be centered on a triangular grid to increase the microlensarray's fill factor, and the exposure field would not necessarily besquare.) The image field radius is denoted h; the exposure field widthis w; the center spacing between spots is d; the center spacing betweenraster lines is δ; and the number of spots per exposure field is N.These quantities, and their relationships defined by the FIG. 7geometry, are tabulated in FIG. 8.

The image field radius is specified as h=0.8 mm (Eq. 8.1 in FIG. 8),implying an exposure field width of w=1.13 mm (Eq. 8.2). The raster linespacing is specified to be approximately 20 nm (δ≅20 nm, Eq. 8.3). (Theimage's minimum spatial period based on the Nyquist frequency limit ofλ/(2 NA) is 111 nm for a wavelength λ of 266 nm and numerical apertureNA of 1.2; thus a 20-nm line spacing is well within the system's opticalresolution limit.) There are N raster lines distributed across the widthdimension w, implying that N≅56569 (Eq. 8.4). The specified number ofspots is increased to N=65536 so that the spots form a 256-by-256 array(Eq. 8.5). This adjustment reduces the raster line spacing to δ=17.3 nm(Eq. 8.6) There are √{square root over (N)} spots distributed across thewidth dimension w; implying that d=4.42 μm (Eq. 8.7).

The projection system's object plane is imaged onto the image plane at25× reduction; thus the 0.8-mm image field radius implies an objectfield radius of 20 mm, and the spot center spacing of 4.42 μm implies amicrolens center spacing of 110 μm.

The modulator switching is assumed to be triggered by a clock withsynchronization frequency f_(sync)=2 MHz (Eq. 8.8). The printing surfaceis assumed to scan a distance δ per clock pulse, implying a scan speedof 34.5 mm/sec (Eq. 8.9). The modulators would not necessarily need toswitch at a 2 MHz rate, but their ON/OFF transitions are controlled with0.5-microsecond resolution. If the switching time is 1 microsecond (2clock pulses), for example, then the distance between switching pointsalong the scan lines would be at least 2δ (i.e., 34.6 nm), but thelocations of the switching points would be specified with a positioningresolution of δ. The switching can be controlled in finer timeincrements (up to the limit of the 80 MHz source repetition rate) toaccurately control gray level and exposure dose.

The area scan rate is 39.1 mm²/sec per exposure field (Eq. 8.10).Multiple projection systems can be used in parallel to achieve a higherscan rate. For example, FIG. 9 illustrates an array of 19 projectionlenses such as lens 901 covering a 300-mm semiconductor wafer 902 (inplan view). The aggregate area scan rate for 19 exposure fields is 742mm²/sec, which equates to 37.8 wafers per hour considering only the scantime. Taking into account the throughput overhead from wafer loading,alignment, and scan reversal, the printing throughput could be expectedto be of order 20 wafers per hour.

In contrast to prior-art immersion lithography systems, the samethroughput level could be maintained at larger wafer sizes by using moreprojection systems. For example, FIG. 10 illustrates 37 projectionlenses 901 covering a 450-mm wafer 902. This system could achieveapproximately the same 20 wafer-per-hour throughput as the 19-lens,300-mm system of FIG. 9. The throughput is limited primarily by theassumed printing grid step (17.3 nm), the SLM clock frequency (2 MHz),the number of modulators and microlenses per SLM (65,536), and thethroughput overhead from non-printing operations (approximately 50%).

Assuming an exposure dose of 30 mJ/cm² (FIG. 8, Eq. 8.11) and opticalefficiency of 25% including transmittance and fill-factor losses (Eq.8.12), the estimated source power requirement per exposure field wouldbe 46.9 mW (Eq. 8.13). (The refractive lens surfaces should beanti-reflection coated to minimize transmittance losses.) The totalpower for a 19-lens, 300-mm system would be 0.89 W, and for a 37-lens,450-mm system it would be 1.73 W. The power from a single laser can bepartitioned (e.g., by means of a Dammann grating) to supply all of theprojection systems.

4. Optical Design

A detailed optical design for the illustrative embodiment is provided inthe '166 application's Computer Program Appendix 4 as a Zemax macro (ZPLfile), which initializes the design in Zemax. (The macro and associatedfiles will be described in section 5.) The design optimization was notperformed in Zemax, but an outline of the design methodology follows.

The optical system is designed from bottom-to-top, using reverse raytracing from the printing surface 105 to the source 101 (FIG. 1A) tooptimize the reverse images of the target exposure points at the source.First, the projection optics are designed to optimize the point-imagingperformance between the object plane and image plane over a range ofwavelengths (e.g., wavelengths 266, 265.98, and 266.02 nm were used forthe illustrative design). In this design phase, optical rays arereverse-traced from a set of design image field points on the imageplane (i.e., the printing surface 105 in FIG. 1B) back to the objectplane (on the bottom of plate 104). A conical fan of rays defined by theimage numerical aperture (NA=1.2) is traced from each image point to theconjugate object spot. (FIG. 2 illustrates meridional limit rays 106 ofone such ray fan intercepting object point 202.) The system designparameters are optimized to minimize the aberrated spot sizes of thereverse point images on the object surface.

In principle, the object surface need not be planar, but a flatnessconstraint is imposed to avoid the complication of fabricatingmicrolenses and SLM components on non-planar substrates. Imagedistortion need not be controlled because the microlens positioning isdetermined to accommodate any distortion. The projection system istelecentric at the image (i.e., the ray fan traced back from each imagepoint covers a conical directional range centered on a chief ray that isnormal to the image plane). The system is also designed to also be atleast approximately telecentric at the object (i.e. the chief rays areapproximately normal to the object plane). This constraint is imposed sothat the microlenses need not operate at extremely oblique incidenceangles, and to avoid extreme distortion of the system's entrance pupil(which would result in highly distorted microlens apertures). The objecttelecentricity is achieved by using an optimization merit function thatincludes penalty terms for non-telecentricity.

At this stage the design does not yet include the microlenses, but therays should be traced back to the microlens plane (the top surface ofplate 104) to ensure that there is no caustic or extreme nonuniformityin the ray distribution over the microlens apertures. A caustic will bemanifested as a local minimum in the meridional ray-intercept coordinateon the microlens plane as a function of image-space ray angle. Acaustic-free, uniform distribution of rays on the microlens plane can beensured by including in the optimization merit function a penalty termrelated to the curvature (nonlinearity) of the microlens-plane raycoordinates versus image-space ray direction for each image point. (Auniform ray distribution can also be achieved by imaging each microlensaperture onto the projection system's entrance pupil by means of afocusing element proximate the microlens focal point. This design optionwill be described more fully in Part 2, section 19.)

The image-field aperture radius is 0.8 mm and the object-field apertureradius is 20 mm, for a 25× demagnification ratio. These aperturedimensions and the axial length of the PL3 . . . PL15 lens group areconstrained during optimization. The axial length controls the maximumlens aperture size, which should be small enough to fit the lens packinggeometry illustrated in FIGS. 9 and 10. The largest lens element, PL10,has a 26.2-mm aperture radius. The object field size should preferablynot exceed this dimension; otherwise it would further constrain thepacking geometry. But within this limitation a large object field hasadvantages in terms of easing microlens fabrication tolerances andenabling the use of a large number of microlenses.

The immersion fluid thickness is 0.5 mm. (The fluid fills the spacebetween the last lens element PL15 and the printing surface, but asnoted previously, a solid cover plate could alternatively be interposedbetween the fluid and the printing surface.) The thickness of themicrolens/SLM plate 104 is 1.56 mm, which is chosen to make themicrolens diameters approximately 100 μm. (As noted previously, themicrolens center spacing is 110 μm.)

After the projection system is defined, the microlens centers aredefined by the centroids of the ray fans on the microlens plane, and theintermediate foci are defined by the ray centroids on the object plane.The collimator 102 (FIGS. 1A, 1B, and 2) is designed so that a reverseray trace of rays through the microlens centers and correspondingintermediate foci converge to the source 101. (The microlenses need notbe defined at this stage because the center rays are undeviated by themicrolenses, although they are deviated by refraction at the microlensplane.) The optical power balance between collimator elements CL1 andCL2 is determined to achieve substantially achromatic reverse imaging ofthe center rays at the source.

With the projection and collimation optics both defined, the microlensesare designed to precisely eliminate point imaging aberrations betweenthe source and the image points at the principal design wavelength of266 nm. As noted in the Overview, the microlenses could be designed asSchupmann doublets to minimize chromatic aberration, but the narrowlaser bandwidth and short microlens focal lengths obviate the need formicrolens achromatization. A single design wavelength thus suffices forthe microlens design.

The microlenses' Fresnel zone patterns are defined by a phase-matchingprocess. For each of a set of design microlens positions, a dense rayfan is reverse-traced from the corresponding image point back to themicrolens surface to define a set of design points (ray-intercept loci)in the microlens aperture. A second ray fan is traced from the source tothe same design points. The microlens-induced optical phase shift ateach design point is defined so that the total source-to-image phase(the sum of the grating phase and the optical phase along the raysegments preceding and following each design point) is constant acrossthe microlens aperture. The grating phase values are fit to a polynomialfunction of position coordinates on the microlens aperture (with thecoordinate origin at the lens center) to define the phase as acontinuous function of position. This process is repeated for each of aset of design microlens positions, and the phase polynomial coefficientsmay themselves be fit to a polynomial function of the microlens centercoordinates (relative to axis 107 in FIG. 1B) to define the gratingphase as a continuous function of microlens center position.

The functional form of the microlens grating phase is outlined in FIG.11. The phase is defined in relation to Cartesian coordinates (x₁, x₂,x₃) with the axis 107 corresponding to the x₁ axis (i.e., x₂=0 and x₃=0on axis 107). The microlenses are on a constant-x₁ plane, and theposition coordinates (x₂,x₃) within each microlens aperture aredecomposed into the microlens center coordinates (x₂′,x₃′) relative toaxis 107 and offset coordinates (x₂″, x₃″) relative to the microlenscenter, Eq. 11.1. The grating phase gp in cycle units (1 cycle=2πradian) is a function of the four coordinates x₂′, x₃′, x₂″ and x₃″, Eq.11.2. (Function arguments are delimited by square braces “[ . . . ]” inFIG. 11.) Based on rotational symmetry of the optical system around axis107, the phase is invariant under rotation of both vectors (x₂′,x₃′) and(x₂″,x₃″) by a common angle θ, Eq. 11.3.

With θ defined as the angle of (x₂′,x₃′) relative to the x₂ axisaccording to Eq. 11.4, the second function argument on the right side ofEq. 11.3 vanishes (Eq. 11.5). Thus, it suffices to define the phasefunction for lens centers in the meridional plane x₃′=0. For any otherx₃′ the phase is defined by Eq. 11.5. gp[x₂′,x₃′,x₂″,x₃″] is invariantunder sign inversion of all of the coordinate arguments (from Eq. 11.3with θ=π); thus it suffices to define gp[x₂′,0,x₂″,x₃″] for x₂′non-negative (Eq. 11.6). The phase function has bilateral symmetryacross the meridional plane x₃′=0 (Eq. 11.7); thus it suffices to definegp[x₂′,0,x₂″,x₃″] for x₃″ non-negative. The function gp[x₂′,0,x₂″,x₃″]may be represented as a multivariate polynomial function of threeparameters x₂′, x₂″ and x₃″, with the choice of monomial terms limitedby the symmetry conditions in Eqs. 11.6 and 11.7. (The phase function'sx₂″ and x₃″ dependence would preferably be modeled using Zernike circlepolynomials for optimal numerical precision, but to accommodate Zemaxlimitations the present design used a standard polynomialrepresentation.)

The Zemax macro in the '166 application's Appendix 4 illustrates thesystem design for 25 representative microlens positions, with thegrating phase for each position defined by an order-14 polynomial in x₂″and x₃″. (In Zemax the grating phase is specified in radians, notcycles.) The polynomial coefficients can be determined by performing aleast-squares fit to a set of rays defined by a square array ofdirection cosines in the image space (truncated to the numericalaperture limit). To minimize numerical precision error, the coordinatesx₂″ and x₃″ should be normalized to the nominal microlens apertureradius (0.05 mm) when calculating the least-squares fit. The fit shouldpreferably be applied not to the grating phase directly, but rather tothe phase gradient, as represented by finite differences betweenadjacent data points on the microlens aperture. (A direct fit to thephase itself will tend to result in steep “walls” in the fitting errorat the aperture boundary, which can cause spurious ray trace errors andpossible manufacturing complications.) The phase polynomial's constantterm is not included as a fitting parameter if a gradient fit isperformed.

In a variation of the above process, the microlens phase maps aredetermined from interferometric measurements of the as-built system inorder to compensate for manufacturing errors. (Analogous methods areused with conventional lens polishing, Ref. 6.) For each of a set oftest microlens positions, a narrow-band radiation beam at the 266-nmdesign wavelength is directed from a corresponding image point throughthe projection system 103 and is interferometrically analyzed todetermine the beam's phase profile across the microlens aperture. A beamis similarly directed from the source point 101 through the collimatoroptics, and its phase profile across the microlens aperture is alsointerferometrically measured. The two phase profiles determine themicrolens phase function in the manner described above, using measureddata in lieu of ray-trace calculations. A sampling of the phasefunctions over a limited number of microlenses can be extended to theentire array by interpolation or polynomial fitting. (The symmetryconditions defined by Eqs. 11.5-7 cannot be assumed to hold in thepresence of manufacturing errors.) With this process, system aberrationscan be reduced to a level that is limited only by the accuracy ofinterferometric measurement and microlens fabrication.

Manufacturing error compensation can alternatively be implemented byfirst assembling the entire optical system, including the microlenses,according to the design specification, measuring the entire system'ssource-to-image wavefront aberrations at selected image points (e.g., bymeans of miniature Shack-Hartmann wavefront sensors positioned below theimage plane), and then designing and constructing a replacementmicrolens array to correct any measured wavefront errors.

In the above-outlined design process, the phase-Fresnel structures (themicrolenses and surfaces 108 and 109 in FIG. 1B) are designed using an“equivalent-element model” in which the structure is represented as afunctionally equivalent, infinitesimally thin phase-shifting layer,which induces a discontinuous optical phase shift between incident andtransmitted beams across a design optical surface. The design surface isproximate the physical phase-Fresnel structure (e.g., at the substrate),and the optical phase shift is equal to the grating phase on the designsurface. The grating phase is defined as a continuous function ofposition on the design surface, which varies by one cycle betweenadjacent Fresnel zone boundaries, and it defines the phase relationshipbetween the incident and transmitted electromagnetic fields extrapolatedto the design surface. (The physical phase-Fresnel structure has finitethickness, but the fields are extrapolated via analytic continuation toa zero-thickness design surface.) After the lens design is completed,the physical phase-Fresnel structure is designed to produce the desiredphase shift in the extrapolated fields. The structure is optimized toachieve high diffraction efficiency and minimal optical scatter withinthe system's field of view. Examples of optimized phase-Fresnelstructures will be illustrated in section 6.

5. The PDF Appendices and Text Files

The Appendices in the '166 application include three ZTG (“Zemax TableGlass”) files (SiO2_266.ZTG, CaF2_266.ZTG, IF132_266.ZTG) and two ZPL(“Zemax Programming Language”) files (SpotScanSystem_reverse.ZPL,SpotScanRMS.ZPL). Appendix 4 (SpotScanSystem_reverse.ZPL) contains adetailed design specification for the FIG. 1 optical system in Zemax(Ref. 1). The ZTG files should be placed in the Zemax “Glass” directory,and the ZPL files should be placed in the “ZPL” directory. Thesedirectory locations are specified in Zemax under theFile—Preferences—Folders tab.

The design is initialized in Zemax by starting with an uninitializeddesign (use File—New to clear the design), invoking the menu commandMacros—Edit/Run ZPL Macros, and selecting SpotScanSystem_reverse.ZPLfrom the pull-down selection list. This sets up a multi-configurationdesign in which each configuration represents a separatefocused-radiation spot and associated microlens position. Twenty-fivemicrolens positions are represented. For each configuration, rays arereverse-traced from the design image point back to the source. After thedesign is initialized, the RMS phase error of the reverse point imagesat the intermediate foci (without aberration compensation) and at thesource (with aberration compensation) can be calculated and tabulated byrunning the SpotScanRMS.ZPL macro.

6. Phase-Fresnel Lenses

Applications of phase-Fresnel lenses for achromatization and forlithography are described in the '843 application and in the prior artreferences cited therein (Miyamoto, cited herein as Ref. 7, and U.S.Pat. Nos. 5,161,057, 5,589,982, and 6,960,773). The '919 applicationalso discusses refractive/diffractive microlens achromats (see the '919application's FIG. 7 and associated Eqs. 7.1-9 in FIG. 4C). Similardesign forms could be used for the projection system's phase-Fresnelsurfaces 108 and 109 in FIG. 1B. U.S. Pat. No. 5,623,365 and Ref. 8describe the use of diffractive lenses for achromatizing lithographyprojection systems, but these disclosures only consider diffractivestructures formed on flat plates. They do not disclose methods formanufacturing achromatizing diffractive lenses on curved substrates orwith accuracies required for DUV lithography.

The phase-Fresnel elements are preferably formed in a high-index opticalcoating material such as HfO₂ deposited on the SiO₂ substrate, ratherthan etching or machining the structures directly in SiO₂. The highrefractive index of HfO₂ (2.1 at 266 nm, versus 1.5 for SiO₂, Ref. 9)results in a much shallower grating profile with less optical scatterinto extraneous diffraction orders relative to an SiO₂ grating. Thephase-Fresnel structures in the projection optics are preferably formedon glass-to-air lens interfaces (not air-to-glass) to minimizetransmission scatter.

The phase-Fresnel gratings can be accurately formed using a multilayerdeposition/etch process similar to that described in the '919application for EUV phase-Fresnel microlenses. The '919 applicationdescribes a process in which multiple Mo/Ru bilayers are deposited on athin, EUV-transparent Si substrate, and are subsequently etched to forma multilevel phase-Fresnel structure, with the Ru layers operating as anetch stop. A similar process is described in U.S. Pat. No. 6,187,211.For DUV lithography at 266 nm, aluminum oxide (Al₂O₃) can be used as anetch stop in conjunction with HfO₂. Al₂O₃ has good transparency and ahigh refractive index (1.7) at 266 nm (Ref. 9), but it should preferablybe used only in very thin layers (e.g., 2 nm) because of the indexdifference between Al₂O₃ and HfO₂.

Other material combinations can be used for the phase-Fresnelstructures. For example, silicon nitride (Si₃N₄), which has a high index(2.2) and only slight optical absorption at 266 nm, could be used incombination with either HfO₂ or Al₂O₃.

A process for manufacturing phase-Fresnel grating structures isconceptually illustrated in FIGS. 12A-12C. A multilayer film stack 1201comprising Al₂O₃/HfO₂ bilayers (e.g., bilayer 1202) and an Al₂O₃ baselayer 1203 (shown in cross-section in FIG. 12A) is deposited on an SiO₂substrate 1204 using a process such as atomic layer deposition (ALD). (Asimilar process has been used to form Al₂O₃/HfO₂ bilayer stacks for CMOSgate dielectrics, Ref. 10, and for RF capacitors, Ref. 11.) The layersare selectively removed by a process such as e-beam lithography, focusedion beam (FIB) machining, or mechanical machining (e.g., diamond-pointturning), FIG. 12B. The last residual layer 1205 of HfO₂ is removed bymeans of a selective etch, which stops at the topmost remaining Al₂O₃layer, FIG. 12C. (A directed ion-beam etch would preferably be used toform vertical or optimally sloped sidewalls.) By this method, thestructure's vertical dimensions are determined by the ALD process, whichcan achieve angstrom-level tolerances on film thicknesses. The etchingor machining process only determines the lateral structure dimensions.

For optimal performance, at least some of the deposited layers forsurfaces 108 and 109 in FIG. 1B may need to have a radial thicknessgradient across the lens aperture. A process such as masked ALD oruniform ALD followed by ion milling may be used to form the thicknessgradient. (Ion milling would not significantly compromise the ALDthickness accuracy if only a small fraction of the ALD-deposited film isremoved.) As an alternative to ALD, a deposition process such as maskedmagnetron sputtering may be used. (A similar process has been used tofabricate graded-thickness EUV mirror coatings, Ref. 12.)

For the axisymmetric surfaces 108 and 109 in FIG. 1B, single-pointdiamond turning can be used to do most of the patterning (FIG. 12B). Thediamond tool would only be used to form shallow steps in a surface whosegeometry has already been accurately determined by spherical polishingand ALD deposition. Thus, precise and accurate cutting can be achievedby using real-time surface metrology (e.g., laser interferometry) on theworkpiece to provide tool feedback control. If the machining issufficiently accurate it may be possible to directly cut optimal,continuous-profile Fresnel facets in a single HfO₂ film without any etchprocesses.

Axisymmetric phase-Fresnel structures can alternatively be formed by an“ion turning” process in which a lathe-type machine operates in vacuumwith a focused ion beam replacing the cutting tool. (This process hasbeen used to manufacture small machining tools, Ref. 13.) Turningprocesses could also be adapted to use e-beam or laser-beam writing forlithographic patterning. Non-axisymmetric phase-Fresnel structures suchthe microlens array on plate 104 can be formed using e-beam lithographyor FIB machining.

As illustrated in FIGS. 3A and 3B, the microlenses have very shallowphase-Fresnel structures, with a profile height of approximatelyone-quarter micron and a minimum facet width of approximately 5 microns.Optical scatter into extraneous diffraction orders is not a significantconcern because most of the scatter can be blocked by apertures at theintermediate foci. The lithographic patterning tolerances would becomparatively loose because of the large (25×) demagnification factor(compared to, e.g., 4× for prior-art mask-projection lithography).

By contrast, the projection system's phase-Fresnel structures are morechallenging. The facet profile heights are also approximatelyone-quarter micron, but the minimum facet width is approximately 2microns for surface 108 (FIG. 5) and 1 micron for surface 109 (FIG. 6).If the system includes polarization-control optics, the surface-109design is simplified because it would operate with substantially TE(transverse-electric) polarization (i.e., linear polarization with theelectric field transverse to the meridional plane at each surfacepoint). Surface 108, on the other hand, would typically need to bedesigned to work with both TE and TM (transverse-magnetic) polarization.

FIG. 13 is an enlarged view of region 13 in FIG. 5, illustrating aFresnel facet structure that could be used near the periphery of surface108. Corresponding design data is tabulated in FIG. 14, and opticalperformance data is tabulated in FIG. 15. For clarity of illustration,the facet aspect ratio is exaggerated by a factor of five in FIG. 13 (asindicated by the scale bars). Fourteen Al₂O₃/HfO₂ bilayers are depositedon the SiO₂ lens substrate, starting with a relatively thick (58-nm)HfO₂ layer. All other HfO₂ layers are 14-nm thick, and all Al₂O₃ layersare 2-nm thick. The first deposited bilayer is unpatterned.

The layer thicknesses are denoted t, and the patterned layers' left andright boundary coordinates are denoted as xL and xR, as illustrated inFIG. 13 for a particular layer. These dimensions are tabulated in FIG.14. The xL and xR coordinates are specified as fractions of the gratingperiod, Λ=2.24 μm. The assumed material refractive indices at the 266-nmdesign wavelength are listed in FIG. 14.

Calculated diffraction efficiencies (averaged over TE and TMpolarization) of all non-evanescent transmitted diffraction orders aretabulated in FIG. 15 for three incidence angles covering the fulloperating angle range. (The grating performance was modeled usingGD-Calc, Ref. 14.) These calculations are for meridional incident rays.The operating incidence range includes skew rays, but the efficiencycharacteristic of a skew ray is nearly the same as its meridionalprojection. Most of the incident energy (about 81%) is diffracted intothe first transmitted order. The total energy scattered into extraneoustransmitted orders amounts to approximately 1% of the first order, andmost of the scattered energy either does not intercept the printingsurface or is broadly dispersed over the surface.

The efficiency data in FIG. 15 contains no phase information. Thegrating's lateral position (along the x direction in FIG. 13) determinesthe phase in the first transmitted order, which should match the designphase as defined by the previously-described equivalent-element model.The phase differs slightly between TE and TM polarizations. (Thedifference is of order 1 milliwave.) For the purpose of lens design, theTE and TM phase can be averaged. The phase difference will be manifestedin the transmitted beam's polarization state, and is taken into accountin the polarization-control optics.

FIG. 16 is an enlarged view of region 16 in FIG. 6, showing anillustrative Fresnel facet design near the periphery of surface 109. Thefacet aspect ratio is exaggerated by a factor of 2.5 in FIG. 16. Thegrating walls are slanted at an angle γ=45° from the substrate normal(although the slant angle appears smaller in the figure due to thedistorted aspect ratio). The grating structure consists of anunpatterned Al₂O₃ base layer and six patterned Al₂O₃/HfO₂ bilayers. TheAl₂O₃ layers are all 2-nm thick, and the HfO₂ thicknesses are allindividually optimized.

The layer thicknesses are denoted t, and the patterned layers' left andright boundary coordinates are denoted as xL and xR, as illustrated inFIG. 16 for a particular layer. (The xL and xR coordinates are projectedonto an x axis in the slant direction, as illustrated.) The design datacorresponding to FIG. 16 is tabulated in FIG. 17. xL and xR arespecified as fractions of the grating period, Λ=1.06 μm.

Optical performance data corresponding to FIG. 16 (TE diffractionefficiency) is tabulated in FIG. 18 for three meridional incidenceangles covering the full operating range. Based on the x sign conventionillustrated in FIG. 16 the grating's operating diffraction order is the−1 order, and the TE efficiency in this order is approximately 82%. Incontrast to the FIG. 13 grating, the FIG. 16 structure's efficiencywould drop off significantly for skew rays (e.g., to around 76%). Inaddition, the TM efficiency would be considerably lower (about 56%), andoptical scatter into extraneous orders would be higher for TM. Thus,polarization control would be required to achieve high efficiency andlow scatter at surface 109.

7. Polarization Control

The projection system may incorporate a form-birefringentpolarization-control grating such as that described in the '843application (see the '843 application's FIGS. 28 and 41-45) and in theprior art cited therein (cited herein as Ref's. 15 and 16). The gratingmay be formed, e.g., on spherical surface 110 of PL11 or on a buriedplanar surface 111 within element PL10 (FIG. 1B). Additionalform-birefringent structures may be provided on the CL2 planar surface(or on the top of plate 104 if the microlenses are formed on CL2). Thelaser source and associated polarizing optics provide substantiallycircularly-polarized radiation to the collimator 102, and theform-birefringent structures operate to make all optical rayssubstantially TE-polarized (i.e., linearly polarized normal to themeridional plane) at the printing surface 105. (Most of the polarizationcontrol functionality can be provided by the projection system, withcomparatively shallow CL2 microstructures functioning to only correctsmall imperfections in the projection system's polarization control.)

Three advantages of polarization control are that (1) it improveslithographic print resolution; (2) it allows use of a high-index,birefringent last lens element PL15 such as sapphire for imaging at ahigher numerical aperture; and (3) it allows the last phase-Fresnelsurface 109 to be designed for substantially TE-polarized light only.

The '843 application illustrates two form-birefringent grating designs,one optimized for uniform transmission efficiency (the '843application's FIG. 44), and one optimized for minimum grating height(the '843 application's FIG. 45). An advantage of the latterconfiguration is that the height minimization condition implies that theTE/TM phase shift (“arg[ρ]”) will be minimally sensitive to the gratingline width (w). The design can alternatively be optimized for uniformgrating height across the lens aperture to simplify the manufacturingprocess. The illustrated design in the '843 application uses Si₃N₄ asthe grating material. Similar structures have been fabricated asvisible-light quarter-wave plates using nonstoichiometric siliconnitride (SiN_(x); Ref. 17). Alternatively, a material such as HfO₂ canbe used, and the grating may be formed on an etch-stop layer such asAl₂O₃. A grating structure with non-uniform thickness can be formed,e.g., by (1) using ALD to deposit a thin Al₂O₃ base layer on an SiO₂substrate, (2) ALD-depositing a thicker, uniform HfO₂ grating layer overthe Al₂O₃, (3) ion-milling the HfO₂ to create the desired thicknessprofile, (4) depositing a second Al₂O₃ layer on top of the HfO₂, (5)patterning the top Al₂O₃ layer using e-beam lithography, and (6) usingmasked ion milling to create the grating spaces (with the top Al₂O₃functioning as a hard mask and the base layer functioning as an etchstop).

8. The Spatial Light Modulator

The focused-radiation spots may be individually modulated by a spatiallight modulator comprising multiple optical modulator elements, one ofwhich is depicted schematically as element 203 in FIG. 2. The '919application illustrates a modulator as element 1105 in the '919application's FIG. 11, and identifies a couple of options for themodulator mechanisms including MEMS-actuated shutters, as disclosed inU.S. Pat. Nos. 6,214,633 and 6,701,039 and Ref's. 18 and 19, or a“Stacked-Grating Light Modulator” (SGLM) as disclosed in U.S. Pat. No.8,687,277. The latter application pertains to reflective modulatormechanisms, but also makes mention of a transmission-type SGLM, which isdisclosed in U.S. Provisional Patent Applications 60/116,074 and60/124,140.

In a preferred embodiment the optical modulators comprise pairedapertures, which are actuated to move in opposite directions asillustrated in FIGS. 19A-19D. FIG. 19A is an enlarged view of region 19Ain FIG. 2. A modulator element 203 on plate 104 is illustrated in its ONstate in FIG. 19A (in a cross-sectional elevation view) and in FIG. 19B(in plan view). Radiation 1901 is directed toward the intermediate focus202 at the projection system's object plane 1902 and transmits throughtwo apertures 1903 and 1904 proximate the intermediate focus. Theapertures are attached to the substrate 104 by means of leaf-springflexures 1905 and 1906. The apertures are mechanically actuated, e.g.,by electrostatic coupling between electrically conductive films 1907 and1908, to control their lateral positional relationship. In the ON statethe apertures are aligned and centered on the intermediate focal point202 to transmit the radiation. In the OFF state, illustrated in FIGS.19C and 19D, the two apertures are laterally displaced in oppositedirections from their ON positions to block the radiation.

A lateral displacement of each aperture by at least half the aperturediameter would suffice to achieve full beam modulation. In the ON statethe aperture boundaries could coincide approximately with the firstdiffraction node of the Airy diffraction pattern at the focal point. Alarger aperture area may be required if the focused beams are highlyaberrated at the intermediate foci to nullify aberration in theprojection system. But in the illustrated embodiment the projectionsystem is diffraction-limited, so the apertures can be close to thetheoretical Airy disk size. The diameter of the first Airy diffractionnode is 1.22λ/NA, where λ is the wavelength (266 nm) and NA is theobject-space numerical aperture, which is smaller than the image-spacenumerical aperture (1.2) by the demagnification factor (25), i.e.NA=0.048. The node diameter is 6.8 micron; thus the modulator couldachieve full modulation with approximately 3.4 microns travel by eachaperture.

9. Wafer Encapsulation and Alignment; Athermalization

State-of-the-art semiconductor manufacture relies on multi-patterning toform sub-wavelength structures with optical lithography. Patternalignment requires accurate overlay control, which can be achieved byusing a backside wafer alignment technique illustrated in FIG. 20. Asemiconductor wafer 2001 (shown in cross-section) is initially processedto form an alignment pattern 2002, in the form of a diffraction grating,on its backside. During subsequent lithography exposure processes, thewafer is vacuum-sealed to a transparent substrate 2003, through whichthe alignment pattern is viewed with alignment optics 2004. (Multipleviewing systems may operate in parallel to provide substantiallyfull-wafer coverage.) The alignment optics comprises a microscopeobjective 2005 through which illumination 2006 is directed (e.g., bydiverting light from a laser source 2007 with fold mirror 2008). Thealignment pattern diffracts the illumination into two reflected beams2009 and 2010 (+1 and −1 diffraction orders), which are collected by theobjective and directed onto an optical detector 2011 conjugate to thepattern. The detector comprises an array of pixel sensor elements, andthe optical interference pattern between the two collected beamsinteracts with the pixel array to form a Moiré signal pattern, which isanalyzed to accurately measure and control the wafer's position relativeto the lithography system. (Small alignment corrections can be effectedby translational motion of the microlens/SLM plate 104 in FIG. 1A, or byactuating the individual microlenses.) Advantages of this method arethat the same backside alignment pattern is used for all process steps;the pattern is unaffected by topside processing and has littlesensitivity to air currents and temperature; the alignment pattern cancover substantially the entire wafer; and the alignment can be monitoredin real time, during the exposure process, at the wafer locationdirectly below each exposure field.

It may also be advantageous to vacuum-seal a thin, transparent coverplate 2012 over the wafer during the lithography exposure process. Thecover plate acts as a solid immersion medium. An immersion fluid wouldbe used between the projection lens 103 and the cover plate, but wouldnot contact the wafer. The cover plate could make it easier to focus theprojection lens on the wafer, because the autofocus mechanism (e.g., anoptical interferometer or capacitive proximity sensor) would rely ondetection of the cover plate's top surface without the complication ofdiscerning focus information on complex wafer topography. Also, byseparating the immersion fluid from the wafer, the impact of particlecontaminants in the fluid on print quality is greatly diminished and thefluid need not be compatible with wafer resist chemistry.

In some applications it may be advantageous to use the cover platevacuum-seal process to perform contact planarization on a deformablephotoresist material. Alternatively, a resist-compatible immersion fluidmay be used for optical coupling between the resist and the cover plate.Some applications might use the cover plate itself as the workpiece. Inthis case a photoresist would be deposited on the cover plate's bottomside, which would subsequently be etched to form useful microstructures.

Aside from alignment and focus, the system may also requireathermalization mechanisms, including sensors and controls, to maintainoptical imaging performance. The sensors may include, for example,micro-optic Shack-Hartmann wavefront sensors that are positioned belowselected image points during the wafer load/unload cycle to detectthermally-induced aberrations. (The Shack-Hartmann sensors may also beused in the manufacturing process to characterize optical aberrationsprior to finalization of the microlens design.) Time-variable imagedistortion can be substantially corrected, e.g., by means ofmicromechanical actuators coupled to individual microlens elements.Also, passive athermalization can be achieved by the choice of lensmaterials and lens housing design.

10. Design Variations

The projection system design illustrated in FIG. 1B can be modified toincrease the numerical aperture and improve image resolution by usingmore optical elements, by using aspheric surfaces (or more phase-Fresnelsurfaces), or by using a high-index glass for the last lens elementPL15. Two glass materials that have been considered for high-NA, 193-nmimmersion lithography are sapphire (Al₂O₃) and lutetium aluminum garnet(Lu₃Al₅O₁₂, also referred to as “LuAg”), but they have not beencommercialized because of the high birefringence of crystalline sapphireand the unacceptable absorption and birefringence of LuAg (Ref's 20, 21,22). However, the absorption and birefringence of LuAg are lower by anorder of magnitude at 266 nm (see FIGS. 5 and 6 in Ref. 20 and FIG. 2 inRef. 21), and the high birefringence of sapphire would not be alimitation if polarization-control mechanisms are employed to ensure TEpolarization on the last lens element (Ref. 22). Furthermore, theeffects of absorption and birefringence would be greatly diminished bythe system's relatively small scale (e.g., 6 mm center thickness forelement PL15 versus over 25 mm for the last lens element in U.S. Pat.No. 8,355,201).

For some applications, Schupmann doublet microlenses similar to thosedescribed in the '919 application could have advantages over the singletdesign described above. The singlet lens form (similar to FIG. 2) anddoublet form (similar to the '919 application's FIGS. 10 and 11) arecomparatively illustrated in FIGS. 21A and 21B. The singlet form (FIG.21A) uses a single microlens element 201 to generate eachfocused-radiation spot by focusing incident radiation throughintermediate focus 202. The radiation may be modulated by modulatorelement 203, and the microlens and modulator may be formed on oppositesides of a solid, transparent plate 104. A Schupmann doublet 2101 (FIG.21B) comprises a similar microlens element 201 of comparatively loweroptical power (i.e., longer focal length) to focus radiation through anintermediate focus 202. But the beam diverging from the intermediatefocus is then further diverged by a second microlens element 2102 havingnegative optical power, so that the emergent beam diverges from avirtual intermediate focus 2103. The optical power can be balancedbetween elements 201 and 2102 to achieve a substantially achromaticvirtual focus 2103. (To some extent, the microlens doublet could also beconfigured to neutralize slight axial chromatic aberration in opticalelements external to the microlens doublet). The beam may be modulatedby a modulator 203 proximate the first (real) intermediate focus 202. Asolid transparent plate 104 may fill the space between microlenselements 201 and 2102, except for the vicinity of modulator 203. (Plate104 may comprise two half-plates that are optically contacted on theintermediate focal plane 2104 after the modulator structures have beenformed.)

FIG. 21B is similar to the '919 application's FIGS. 10 and 11, but witha couple of differences. The first microlens 201 in FIG. 21B is convexand the second microlens 2102 is concave (not vice-versa, as in the '919application) because the microlens material has a refractive indexgreater than 1. Also, the space between the microlenses may be solid(e.g., SiO₂, not vacuum as in the '919 application). The geometricrelations illustrated in the '919 application's FIG. 8A and Eq. 8.1 inthe '919 application's FIG. 4C assume that the medium between themicrolenses is vacuum. With a solid medium, the axial distances betweenthe microlenses and the intermediate focal plane 2104 are increased by afactor of the medium refractive index.

The Schupmann microlens configuration has potential advantages inaddition to achromatic performance. Dividing the microlens functionalitybetween two elements of comparatively lower optical power can result ina more manufacturable microlens design. The first element 201 cancorrect aberrations between the source 101 (FIG. 1) and the intermediatefocus 202, while the second element 2102 corrects aberrations betweenthe intermediate focus and the printing surface 105, resulting in asubstantially aberration-free intermediate focus (except for chromatic).This can be advantageous for the modulator design. The comparatively lowoptical power of element 201 results in a commensurately largediffraction-limited focus 202, which would be a disadvantage ifshutter-type modulators such as the FIG. 19A-19D system are used. But itwould also reduce the beam divergence angle at the intermediate focus,which would be advantageous if diffractive modulator elements (describedbelow) are used.

The phase-Fresnel microlenses (either singlet or Shupmann doublet) andthe projection system's phase-Fresnel optics can take a variety ofstructural forms other than those described above. FIGS. 22A-22Dillustrate several variant microlens forms. FIG. 22A shows across-sectional profile 2201 of a conventional phase-Fresnel surface,which is blazed for optimum first-order efficiency at a particulardesign wavelength (e.g., 266 nm). This form is similar to microlens 201in FIG. 3A, but with the profile height exaggerated. The multilevelstepped profile illustrated in FIG. 12C is an approximation to an idealsloped facet profile illustrated in FIG. 22A.

The surface profile shape in FIG. 22A is defined so that the gratingphase (i.e., the optical phase discontinuity from the incident to thetransmitted beam) varies by one cycle between adjacent facets on thedesign surface in the equivalent optical model, and the profile stepsbetween facets induce a one-cycle optical phase discontinuity in thetransmitted beam. More generally, the facet boundaries on aphase-Fresnel structure can be placed at any position (not just atone-cycle intervals of the grating phase), and the profile steps betweenfacets can induce optical phase discontinuities equating to any integernumber of cycles. For example, FIG. 22B illustrates a phase-Fresnelprofile 2202, which is similar to profile 2201 except that the centerprofile step has been omitted.

In FIG. 22C, the design grating phase varies by two cycles betweenprofile steps on profile 2203, and the steps induce a two-cycle opticalphase discontinuity. Profile 2203 represents a phase-Fresnel structurethat is blazed in the second diffraction order at the primary designwavelength (e.g., 266 nm), and is simultaneously blazed in the firstorder at approximately twice the primary wavelength (e.g., 532 nm), asdisclosed in U.S. Pat. No. 5,589,982. (For near-normal incidence theorder-m blaze wavelength is approximately equal to |n−n′|h/m, where h isthe profile step height, and n and n′ are the optical medium refractiveindices on either side of the profile. n and n′ may be functions ofwavelength.) A high-order phase-Fresnel structure can be useful forapplications requiring multi-wavelength operation, such as absorbancemodulation optical lithography, two-color lithography, ormulti-wavelength scanning confocal microscopy.

The microlenses can alternatively be continuous-profile, refractive lensstructures, as illustrated by profile 2204 in FIG. 22D. Any of theprofile forms in FIGS. 22A-22D, including the refractive microlensprofile 2204, can be constructed as an approximately equivalentmulti-step profile, which is manufactured by a process such as thatillustrated in FIGS. 12A-12C.

A phase-Fresnel transmission lens can be roughly characterized as adiscontinuous optical surface that induces integer-cycle optical phasediscontinuities at the surface profile steps. This characterization isbased on geometric optics concepts, but the types of diffractivestructures illustrated in FIGS. 13 and 16, which are optimized usingaccurate electromagnetic simulations, have at least a coarse resemblanceto the Fresnel facets illustrated, e.g., in FIG. 22A. The microlens 201in FIGS. 2 and 3A is illustrated as a phase-Fresnel structure, but canin general be any type of refractive and/or diffractive optical focusingdevice.

Diffraction optics can also be used as modulator elements, as analternative to the shutter-type device illustrated in FIGS. 19A-19D.Rather than blocking radiation with a non-transmitting shutter in themodulator's OFF state, the radiation can be directed out of the imagefield by means of a transmitting, scattering surface such as a phasediffraction grating configured to extinguish the zero transmitted order.Proximate transmission gratings can be used in combination to achievefull beam modulation with very small (e.g., submicron) mechanicalmotion, as disclosed in U.S. Provisional Patent Applications 60/116,074and 60/124,140. An optical modulator 203 of this type is illustrated incross-section in FIGS. 23A and 23B.

A first lamellar grating 2301 comprises a high-index optical medium suchas HfO₂ deposited on a substrate 2302 such as SiO₂ (perhaps with aninterfacial etch-stop layer such as Al₂O₃). The first grating isproximate a second grating 2303 comprising, e.g., HfO₂ on a SiO₂superstrate 2304. In the FIG. 23A configuration, representing thedevice's OFF state, incident radiation 2305 transmits through the deviceand the two gratings operate conjunctively to substantially extinguish atransmitted beam's zero order 2306, diverting the radiation intodiffraction orders such as the +1 and −1 orders (2307 and 2308). In theON state (FIG. 23B) one or both gratings are laterally translated tochange their positional relationship, and the gratings operate totransmit most of the incident radiation 2305 into the zero order 2306,with comparatively little energy going into the extraneous orders 2307and 2308.

A grating modulator such as that illustrated in FIGS. 23A and 23B hasthe advantage that it can achieve full modulation with very small (e.g.,submicron) translational motion. However, it is difficult to achieve ahigh extinction ratio over a large range of incidence angles, so thedevice typically requires highly collimated radiation, implying acomparatively large beam aperture. An advantage of the Schupmann doubletsystem (FIG. 21B) for this type of modulator is that the low power ofthe first microlens 201 results in lower beam divergence at themodulator 203 (compare FIGS. 21A and 21B).

The microlens/SLM configurations illustrated in FIGS. 2, 21A and 21B canbe modified to use reflective modulators, such as that of U.S. Pat. No.8,687,277, or other prior-art reflective modulators similar to TexasInstruments' Digital Micromirror Device (DMD, U.S. Pat. No. 5,061,049)or Silicon Light Machines' Grating Light Valve (GLV, U.S. Pat. No.5,841,579). This can be accomplished by folding the light path before orafter the reflective modulator elements. An illustration of this designalternative is provided in the '407 application's FIG. 26.

The projection system design detailed in FIGS. 1A and 1B and in the '166application's Computer Program Appendix 4 is premised on the use of a266-nm frequency-quadrupled diode laser with a wide spectral bandwidth(50 pm, compared to less than 1 pm for line-narrowed excimer lasers),necessitating the use of an achromatic projection lens. However, analternative 266-nm, continuous-wave laser presently available from theOXIDE laser company (Ref. 23) has a bandwidth of only 0.002 pm, whichwould eliminate the need for achromatization. The OXIDE laser couldsignificantly simplify the projection optics design by eliminating theneed for phase-Fresnel lens surfaces 108 and 109 (FIG. 1B). It may stillbe advantageous to use such structures to provide the functionality ofaspheric lens surfaces, but they would not be needed forachromatization.

A phase-Fresnel lens surface that operates only to provide asphericpower could have a much coarser grating pitch (making it moremanufacturable and efficient) because most of the lens's optical powercan be in its spherical refracting surfaces. For example, thephase-Fresnel structure's line density (i.e., the grating phasegradient) can be constrained to be zero at the lens edge. (By contrast,the line density at the edge of element 108 is approximately 500/mm, andthat of element 109 is about 1000/mm.)

The OXIDE laser is available with an output power of up to 2 W. Designconcepts for highly efficient continuous-wave DUV lasers with muchhigher power are under development by William F. Krupke (WFK Lasers),e.g., see U.S. Pat. No. 7,283,576, “Optically-pumped DUV atomic vaporlasers”.

Phase-Fresnel lens surfaces could also be used to achromatize projectionoptics for an excimer laser such as a 193-nm argon fluoride (ArF) laseror a 157-nm fluorine (F2) laser, eliminating the need for laser linenarrowing. The F2 laser was abandoned as a successor to the ArF laserfor mask-projection lithography, primarily due to the large volume ofhigh-quality CaF2 lens material required, but a maskless, spot-scanningsystem would use much smaller lens elements of lesser volume. The smalllens scale would also greatly reduce the impact of lens birefringence,and the spot projection optics could be designed to neutralize anyremaining birefringence effect (e.g., by means of polarization-controlelement 3106 in the '843 application's FIGS. 31A and 31B). Thus, the F2laser may be much more practical for scanned-spot-array lithography.

An excimer laser's low repetition rate (e.g. 6 kHz) would beimpractically slow with the 17.3-nm square printing grid of thepreceding illustrated embodiment (FIGS. 7 and 8), but PART 2 willdescribe an alternative print strategy that overcomes this limitation.

Part 2 11. Nodal Line Printing

The preceding sections described a spot-scanning system that is designedto provide optimal point-imaging resolution, but lithography systemstypically only require good line-imaging resolution. A perfectly uniformline image can be synthesized from a periodic sequence of discrete pointexposures along the line if the period is smaller than the opticalresolution limit, λ/(2 NA), where λ is the wavelength and NA is thenumerical aperture in the scan direction. The point exposure spots couldbe formed with a spot-scanning system using a pulsed laser illuminationsource, but throughput would be limited by the illustrated embodiment's17.3-nm scan step, which is unnecessarily small. Throughput can beincreased by an order of magnitude by using a scan step close to theλ/(2 NA) resolution limit.

The cross-scan grid step (line pitch) can also be much greater than 17.3nm, with no compromise in printing resolution, by employing analternative “nodal line printing” method in which zero- or low-intensitydiffraction nodes are used to print narrow lines at a low exposurethreshold. The system aperture is configured to produce a periodicpattern of parallel, linear diffraction nodes, and the cross-scan gridstep is equal to the nodal line separation. Depending on the apertureapodization the cross-scan step would be of order λ/NA, where NA is thenumerical aperture in the cross-scan direction. This is much larger than17.3 nm, and the coarser line density would increase throughput by anadditional order of magnitude relative to the previous illustratedembodiment. Coarse-pitch line patterns can be interleaved, usingprior-art multi-patterning methods or dual-wavelength recordingprocesses, to form high-density line patterns far exceeding the opticalresolution limit.

Other design modifications (aperture narrowing in the scan direction,reduced microlens dimensions) could further improve throughput, enablinghigh-throughput printing with the limited repetition rate of an excimerlaser such as a 193-nm ArF laser or a 157-nm F2 laser. Aside from theoptical resolution benefit from a smaller wavelength, the laser'scomparatively low repetition rate (e.g., 6 kHz) would reduce data flowrequirements for the optical modulators, and could enable additionalmodulation capabilities including dynamic gray-level and spot-centeringcontrol. These and other related design alternatives are described morefully in the following sections.

12. Aperture Form

A nodal line pattern can be produced in the focused-radiation spots byusing a substantially rectangular aperture stop. The precise sense inwhich the stop is “rectangular” is defined as follows. (The followingprescription of the limiting-aperture geometry represents a preferredaperture form, but is not necessarily the only aperture form that canproduce nodal lines.)

Spatial positions proximate a focused-radiation spot will be defined bya vector x, which has coordinate projections x₁, x₂, and x₃ relative torespective orthonormal basis vectors ê₁, ê₂, and ê₃, where ê₁ isdirected normal to the image plane and ê₂, and ê₃ are parallel to theimage plane with ê₂ directed in the spot's scan direction relative tothe printing surface and ê₃ directed in the cross-scan direction; seeFIG. 24, Eq. 24.1. (Normally the printing surface is scanned relative toa stationary spot array, so the surface scan direction is −ê₂.) Anoptical ray intercepting the image plane in a recording medium ofrefractive index n is characterized by an index-normalized wave vector uwith coordinate projections u₁, u₂, and u₃, which has magnitude n; Eq.24.2.

For an optical system with a circular aperture stop, the image-planeprojections of the aperture-transmitted rays' u vectors in the imagespace are limited to a circle of radius NA, the numerical aperture (Eq.24.3). For a rectangular aperture inscribed in a circular aperturelimit, u₂ and u₃ are individually limited in magnitude to numericalaperture limits NA₂ and NA₃, respectively, where the magnitude of vector(NA₂,NA₃) is equal to the circular limit NA; Eq. 24.4. FIG. 25illustrates the geometry of the circular aperture 2501 and the inscribedrectangular aperture 2502. The latter aperture is “rectangular” in termsof the aperture-transmitted rays' u₂ and u₃ projections at the imageplane, but the actual physical aperture stop that has thischaracteristic might not be exactly rectangular.

The aperture is narrowed in the ê₂ direction, i.e., NA₂ is significantlysmaller than NA₃ as illustrated in FIG. 25. The narrowed aperture wouldelongate the diffraction-limited focus spot on the image plane, asillustrated in FIG. 26. (Focus spot 702 on printing surface 105 iselongated into spot 2601 as a result of the aperture narrowing; cf. FIG.7.) But the spots are only elongated in the scan direction ê₂; thecross-scan optical resolution in sectional plane 2602 is not impaired.Thus, the system would be suitable for exposing line features aligned inthe scan direction. The reduced optical resolution in the scan directionwould allow for a larger scan step per laser pulse, so printingthroughput would be increased.

The rectangular aperture shape allows multiple projection systems to bejuxtaposed with greater packing density, as illustrated in FIG. 27.(Compare to the circular aperture geometry in FIG. 9.) Also, theaperture narrowing increases the number of projection apertures that cancover the wafer 902. For example, circular aperture 2701 is truncatedalong the scan direction ê₂ to form a narrowed aperture 2702, which isclosely juxtaposed to adjacent aperture 2703. This allows moreprojection systems to cover the wafer 902. The microlens/SLM plate 104(FIG. 1B) would also need to be truncated to achieve the higher packingdensity, but the individual microlens apertures can themselves betruncated in the scan direction and juxtaposed more compactly. (Themicrolenses' aperture shapes would generally approximately match theshape of the aperture stop.) The total number of microlenses andfocused-radiation spots per projection system need not be reduced byaperture narrowing, so printing throughput can increase in proportion tothe number of projection systems.

A further advantage of aperture narrowing is that it would reduce theeffect of polarization on the nodal line contrast. Thepolarization-control structure on surface 110 or 111 (FIG. 1B) may notbe needed. It may suffice to simply direct substantially linearlypolarized light into the system, with the polarization direction normalto the cross-scan direction. This could be advantageous for the SLMdesign if a grating modulator such as that illustrated in FIGS. 23A and23B is used, because the grating elements would only need to beoptimized for linear polarization. (Form-birefringentpolarization-control surfaces could follow the SLM elements tocompensate for polarization effects in the projection optics.)

13. Pupil Function

In high-resolution line printing operations the system aperturetransmittance may be controlled to effect apodization or dipoleillumination. The aperture transmittance is described in terms of theresulting electric field distribution inside the recording mediumproximate a focused-radiation spot. The field comprises a superpositionof plane waves of the form A

${\exp\left\lbrack {\frac{2\; \pi}{\lambda}{u \cdot x}} \right\rbrack},$

where x is a spatial position vector (Eq. 24.1), u is anindex-normalized wave vector (Eqs. 24.2), λ is the vacuum wavelength,and A is a field amplitude vector orthogonal to u—see FIG. 28A, Eqs.28.1. (Vectors are represented in bold type and function arguments aredelimited by square braces “[ . . . ]”.) The total electric field E atpoints x proximate a focus spot has a Fourier integral representation asdescribed by Eq. 28.2. A is represented as a function of only twowave-vector components, u₂ and u₃, because Eqs. 24.2 implicitly defineu₁ as a function of u₂ and u₃; see Eq. 28.3.

We are only concerned with the field in the focal plane, x₁=0, so Eq.28.2 is simplified to Eq. 28.4. (The focus spot's geometric focal pointis at x₂=0=x₃.) The field is constrained by polarization-control opticsto be orthogonal to the cross-scan direction (Eq. 28.5). Based on Eqs.28.1 and 28.5, the amplitude vector A can be represented by Eq. 28.6,where the scalar factor P[u₂,u₃] represents a “pupil function” in thevicinity of the geometric focus. With substitution of Eqs. 28.6 and 28.3in 28.4, the coordinate projections of E reduce to Eqs. 28.7-9.

Typically, the E₁ field component is insignificant due to the u₂ factorin the Eq. 28.9 integral. An accurate characterization of the imagefield E would require both Eqs. 28.8 and 28.9, but a simplified scalartheory can be used by considering only E₂ (Eq. 28.8) and neglecting E₁.Furthermore, we will consider pupil functions P[u₂,u₃] that aremultiplicatively separable into factors P₂[u₂] and P₃[u₃], for whichE₂[x₂, x₃] similarly separates into factors E_(2,2)[x₂] and E_(2,3)[x₃](Eqs. 28.10-11). We will primarily be concerned with the field'samplitude distribution E_(2,3)[x₃] in the cross-sectional plane x₂=0, asdefined by the second of Eqs. 28.11.

With P₃[u₃] constant and equal to 1 over the full aperture, E_(2,3)[x₃]has the functional form given in Eqs. 28.12 in FIG. 28B. The pupilfunction P₃[u₃] is illustrated in FIG. 29 (solid line 2901), and thefield intensity |E_(2,3)[x₃]|² is illustrated in FIG. 30 (solid line3001). The field has diffraction nodes at x₃=½mλ/NA₃ for non-zerointegers m (as indicated in the bottom line of Eqs. 28.12).

The pupil function can be apodized to suppress diffraction tails,although this will broaden the central diffraction peak. For example,the dashed lines in FIGS. 29 and 30 represent an apodized pupil functionP₃[u₃] (line 2902) and the corresponding field intensity |E_(2,3)[x₃]|²(line 3002). (The illustrated intensity plots are peak-normalized.)These functions are defined by Eqs. 28.13. The diffraction nodes are atx₃=mλ/NA₃ for non-zero integers m.

Focus spots characterized by FIG. 30 could be used to print periodicline patterns with a pitch of order λ/NA₃, and multiple such patternscan be interleaved or overlaid, using prior-art multi-patterningtechniques, to form narrow, densely-spaced line patterns below theoptical resolution limit.

Very narrow sub-resolution resist lines can be formed in a singleexposure step by using an alternative pupil function illustrated in FIG.31. A half-cycle (π-radian) phase shift is applied to half of theaperture to create a transmittance sign discontinuity at the aperturecenter (solid curve 3101). The resulting field intensity plot (curve3201 in FIG. 32) has a diffraction node at the center of the focus spot.The functional form of these curves is defined by Eqs. 28.14. (“sgn” isthe sign function, which is +1 for positive arguments and −1 fornegative arguments.) The spot pattern has nodes at all integer multiplesof λ/NA₃, including zero.

Narrow resist lines of width w can be exposed at intensity threshold tnear the central node, as illustrated in FIG. 32, and multiple lines canbe scanned by different spots to form a periodic line pattern with pitchλ/NA₃. A second scanned exposure can be performed using the apodizedpupil function 2902 and corresponding exposure pattern 3002 (dashedcurves in FIGS. 31 and 32) to selectively trim the line patterns. (Thisis a “coarse trim,” which may need to be followed by a higher-resolutionpatterning step to complete the trim process.) A sparse array of linepatterns (or spaces, using a negative resist) formed in this manner maybe useful for trimming an underlying dense line pattern oriented in thecross-scan direction.

Alternatively, a resist freeze process or multiple litho-etch steps maybe used to interleave multiple sparse line patterns to form a densearray with pitch much smaller than λ/NA₃. A sidewall spacer method canbe used to achieve further pitch division. (These types of prior-artmulti-patterning methods are discussed in Ref. 24.) Section 20 describesan alternative dual-wavelength recording process in which exposurepatterns 3002 and 3201 are simultaneously illuminated with differentwavelengths to expose high-density line patterns without intermediateprocessing steps.

The pupil function can be implemented either on an optical surfaceproximate the projection system's aperture stop, or on the microlenssurfaces. It may be advantageous to image each microlens aperture ontothe projection system's entrance pupil by means of a weak focusingelement proximate the microlens focal point. Each microlens would thenfunction as a pupil-defining aperture stop for the corresponding imagepoint. (This design option will be discussed more fully in section 19.)

FIGS. 33 and 34 illustrate two alternative microlens forms that could beused to create the nodal lines. FIG. 33 illustrates arectangular-aperture refractive lens 3301 having a continuous surfaceprofile, except for a step discontinuity 3302 along the scan directionê₂. The step induces a half-cycle phase discontinuity in the transmittedelectromagnetic field (equivalent to a sign change of the field acrossthe step, as illustrated by the sign discontinuity in plot 3101 of FIG.31). FIG. 34 illustrates a rectangular-aperture phase-Fresnel lens 3401,which has a functionally similar step discontinuity 3402 in its surfacetopography. The sloped Fresnel zone surfaces illustrated in FIG. 34 canbe approximated by stepped, planar surface structures that are formedusing an ALD/e-beam process similar to that illustrated in FIGS.12A-12C.

The surface forms illustrated in FIGS. 33 and 34 could alternatively beused for a large-scale optical surface in the projection optics,although a large phase-Fresnel element would have many more Fresnelfacets than the few illustrated in FIG. 34. A structure similar to FIG.34 can be formed on a projection lens surface by first forming anaxisymmetric phase-Fresnel surface pattern via ALD and ion turning, andthen ion milling half of the lens aperture to form the half-cycle phasestep.

The lens forms illustrated in FIGS. 33 and 34 create a double-peakedimage-plane intensity pattern 3501 (i.e., |E₂[x₂,x₃]|²), as illustratedin FIG. 35, which exhibits nodal diffraction lines such as lines 3502,3503 and 3504 in the scan direction. Plot 3201 in FIG. 32 represents across section of pattern 3501.

The nodal line exposures and trim exposures can be performed withseparate microlens groups within a projection system, or with entirelyseparate projection systems. The apodized pupil function 2902 in FIG. 31can be implemented by means of an optically absorptive layer formed on alens surface, or by a diffractive structure, which attenuates the zeroorder while diverting the first and higher order out of the opticalfield of view. It would also possible to implement either of the pupilfunctions 2902 or 3101 in FIG. 31 with an optical structure similarlyformed on a mirror surface.

The side diffraction nodes in plot 3201 of FIG. 32 are much flatter thanthe central node, providing some latitude for variation of the linepitch from the exact period λ/NA₃ defined by the node spacing. (Thischaracteristic will be useful for spot centration control, discussed insection 19, which enables printing of slightly non-periodic ornon-straight line patterns.) Furthermore, the secondary side peaks inplot 3201 (which are noticeable as small bumps in the FIG. 35 area plot)can be suppressed by replacing the discontinuities in pupil function3101 of FIG. 31 with finite-slope apodized transitions. This wouldsomewhat broaden the two central peaks, but it could allow much greaterlatitude in the line pitch. With sufficient apodization, the secondarypeaks can be substantially eliminated and the line pitch need only begreat enough to clear the central peaks. For example, the pupil functionrepresented by plot 3102 (dotted line) in FIG. 31 produces thecorresponding field intensity plot 3202 in FIG. 32. The functional formof these plots is defined by Eqs. 28.15 in FIG. 28B. The intensity plothas discrete nodes at integer multiples of 3/2λ/NA₃, but the secondaryside peaks are reduced from about 9% to 0.6% of the central peaks in the|E_(2,3)[x₃]|² plot (curve 3202 versus curve 3201).

Higher-density line patterns can be exposed by using dipoleillumination, e.g., as illustrated in FIGS. 36 and 37 (although the linepitch is much more constrained with this approach). The middle half ofthe pupil is masked and one of the remaining sub-apertures isphase-shifted by one-half cycle, resulting in the pupil function P₃[u₃]as illustrated by solid curve 3601 in FIG. 36. The correspondingintensity plot, curve 3701 in FIG. 37, has multiple prominent nodes on aperiodic grid of pitch ⅔λ/NA₃; Eqs. 28.16. This exposure pattern wouldprint periodic lines, which may be trimmed with a second exposure usingthe pupil function and spot profile represented by dashed curves 3602and 3702; Eqs. 28.17. (Spot profiles 3701 and 3702 have coincidingdiffraction nodes where x₃ is any integer multiple of ⅔λ/NA₃, except forthe diffraction peak in profile 3702 at x₃=0.) The trim exposure canalternatively be done with an apodized pupil function, e.g., asrepresented by dotted curve 3603 in FIG. 36 and dotted spot profile 3703in FIG. 37; Eqs. 28.18.

14. Interleaved Raster Scan

In the FIG. 7 embodiment rows of focused-radiation spots scan rasterlines with raster line spacing δ much smaller than the spacing d betweenspot rows (e.g., δ=17.3 nm and d=4.42 μm in FIG. 8). The number of spotsper row is d/δ (e.g., 256 in the preceding design example). The nodalline printing method uses a much coarser raster line spacing, e.g.δ=λ/NA₃ for the spot intensity profile 3201 in FIG. 32. For example,with λ=266 nm and NA₃=1.2 the line spacing would be δ=222 nm. Assuming aspot row spacing of d=4.42 μm, the same as in FIG. 8, the number ofspots per row would be only d/δ=20. With a 25×-reduction projectionlens, and assuming a square lens centering layout, the correspondingmicrolens row length in the object space would be only 4.42 μm×20×25=2.2mm, much less than the available 40-mm object field diameter in the FIG.8 design.

The available field area can be more fully utilized by using analternative “interleaved raster scan” method described in U.S. Pat. No.6,498,685. See the '685 patent's FIGS. 35A, 35B, and associateddiscussion beginning at column 34, line 49. (The particular methoddescribed herein is termed a “transitionless scan” in the '685 patent.)This method is primarily applicable with a pulsed light source such asan excimer laser; but a variant of the method described in section 16can work with a continuous source. The '685 disclosure pertained to aspot-scanning system using an image-plane microlens array, but themethod is applied here with an object-space microlens array.

With the raster scan method illustrated in FIG. 7, each raster line isscanned by a single focused-radiation spot, e.g., spot 702 scans rasterline 703. (The diffraction-limited extent of spot 702 covers multipleraster lines, but only that spot's geometric image point scans line703.) With the interleaved raster scan, multiple spots scan each rasterline. Each spot prints a series of sparsely centered “dots” as theillumination is pulsed, and the dot patterns printed by different focusspots on the same line are interleaved to form a uniform, dense patternof evenly spaced dots. (A “dot” is the diffraction-limited exposurepattern formed by a single focused-radiation spot during a singleillumination pulse.)

The scan process is illustrated schematically in FIGS. 38-44. Notationalsymbols and equations associated with the scan process are tabulated inFIG. 38. (There are some notational differences from '685; e.g., theterm “Spot” herein replaces “Pixel” in '685 but has a similarcorrespondence to a microlens and optical modulator element).

In FIG. 39A a raster line 3901 on printing surface 105 is covered by arow 3902 of focused-radiation spots labeled as Spot[ . . . , i_(spot)],i_(spot)=0, 1, . . . , N_(spot/row)−1, where N_(spot/row) is the numberof spots in the row. (The spot index i_(spot) is associated with acorresponding microlens and modulator element. The “ . . . ” precedingi_(spot), and similarly appearing in other indexed expressions,represents additional contextual indices, which will be describedbelow.) The spots are represented schematically by the large circles inFIG. 39A, in which N_(spot/row)=3. (Each spot is a diffraction-limitedfocused-radiation point centered on the corresponding circle.) Theraster line comprises image dots represented by “+” marks for unexposeddots, and by small circles for exposed dots. The sequential illuminationpulses in multiple line-scan operations are designated asPulse[i_(line),i_(pulse)], i_(line)=0, 1, . . . , N_(line/row)−1,i_(pulse)=0, 1, . . . , N_(pulse/line)−1. Index i_(line) labels the linescans, and i_(pulse) labels the pulses within each line scan.N_(line/row) is the number of raster lines scanned by each spot row(i.e., the total number of line-scan operations performed in a printingoperation), and N_(pulse/line) is the number of illumination pulses perline scan. The dot exposed by Spot[ . . . , i_(spot)] andPulse[i_(line),i_(pulse)] is labeled as Dot[ . . . , i_(spot), i_(line),i_(pulse)]. One particular spot, Spot[ . . . , 1], is highlighted inFIGS. 39A-39C and the dots printed by this spot are indicated byfilled-in circles. FIG. 39A illustrates the exposure pattern immediatelyafter Pulse[0,0] (the first exposure pulse), when each spot has exposedone dot.

FIG. 39B illustrates the exposure pattern after Pulse[0,1] (the secondpulse). The spots are intensity-modulated by an object-plane spatiallight modulator in synchronization with the scanning mechanism and theillumination pulse generation. For the purpose of illustration, FIGS.39A-39C show the dot exposure pattern with all of the modulators held intheir ON states. The scan direction of the focused-radiation spotsrelative to the printing surface is indicated by vector ê₂ (directedleft-to-right) in FIG. 39B, and ê₃ points in the cross-scan direction.(In practice, the spots may be substantially stationary while theprinting surface is physically scanned from right to left, asillustrated by the right-to-left direction arrow 701 in FIG. 7.) Theimage surface points are parameterized by x₂, x₃ position coordinateswith the x₂ and x₃ axes aligned to ê₂ and ê₃, respectively. The printinggrid step in the ê₂ direction (i.e., the dot center spacing) isindicated as G₂ in FIG. 39B, and the x₂ step per pulse isN_(dot/pulse)·G₂ where N_(dot/pulse) is an integer. The x₂ center offsetbetween focused-radiation spots is N_(dot/spot)·G₂ where N_(dot/spot) isan integer. (In FIGS. 39A-39C N_(dot/pulse)=3 and N_(dot/spot)=5.)

FIG. 39C illustrates the print pattern after a large number of pulses.The spots' exposure patterns are interleaved to expose all dots thathave been traversed by all spots in the row, without anymultiple-exposed dots. (The dots' geometric image points do not overlap,although each dot's diffraction-limited extent will generally overlapadjacent dots). Complete and non-redundant exposure coverage will beachieved, as illustrated, if N_(dot/pulse) is equal to N_(spot/row), andif N_(dot/pulse) and N_(dot/spot) are relatively prime, Eqs. 38.1 and38.2 in FIG. 38. (In Eq. 38.2 “GCD” denotes the greatest commondivisor.)

In some designs it may be advantageous to print separate, interleavedsets of dots on a raster line with separate spot rows or separate linescans covering the same line. In this case G₂ is defined as the x₂ gridstep of the dots printed by a single spot row in a single line scan. Thecomposite, interleaved exposure pattern will have a dot spacing of G₂/nfor some integer n greater than 1.

N_(dot/spot) should be sufficiently large to accommodate the microlenscenter spacing in the object space. Additional raster lines are scannedby similar spot rows in an echelon configuration, as illustrated in FIG.40. The spot rows in the echelon form an “echelon block” 4001. The rowsare labeled as Row[ . . . , i_(row)], i_(row)=0, . . . ,N_(row/block)−1, where N_(row/block) is the number of spot rows perblock. (Row[ . . . , 0] corresponds to spot row 3902 in FIG. 39A, whichscans line 3901.) Spot i_(spot) in Row[ . . . , i_(row)] is labeled asSpot[ . . . , i_(row), i_(spot)], and the dot exposed by this spot inPulse[i_(line),i_(pulse)] is labeled as Dot[ . . . , i_(row), i_(spot),i_(line), i_(pulse)]. The x₂ offset between echelon rows isN_(dot/row)·G₂, where N_(dot/row) is the echelon step, in dot units, perrow: N_(dot/row)=N_(dot/spot)·N_(spot/row), Eq. 38.3.

Multiple echelon blocks are combined to cover a projection system'simage field 4101, as illustrated in FIG. 41. The echelon blocks arelabeled Block[ . . . , i_(block)], i_(block)=0, 1, . . . ,N_(block/field)−1, where N_(block/field) is the number of blocks perimage field. (Block[ . . . , 0] corresponds to block 4001 in FIG. 40.)Spot row i_(row) in Block[ . . . , i_(block)] is labeled Row[ . . . ,i_(block), i_(row)]; focused-radiation spot i_(spot) in this row islabeled as Spot[ . . . , i_(block), i_(row), i_(spot)]; and the dotexposed by this spot in Pulse[i_(line),i_(pulse)] is labeled as Dot[ . .. , i_(block), i_(row), i_(spot), i_(line), i_(pulse)].

The printing grid step (raster line center spacing) in the ê₃(cross-scan) direction is indicated as G₃ in FIG. 41 (equivalent to δ inFIG. 7). Echelon rows have an x₃ offset (center spacing) of G₃, and thex₃ offset between echelon blocks is N_(row/block)·G₃. The total numberof spot rows per image field isN_(row/field)=N_(row/block)·N_(block/field), Eq. 38.4.

In some designs it may be advantageous to print separate, interleavedsets of raster lines with overlapping image fields or separate linescans. In this case G₃ is defined as the center spacing of the rasterlines scanned over a single image field in a single line scan. Thecomposite, interleaved exposure pattern will have raster line spacing ofG₃/n for some integer n greater than 1. Also, in FIG. 41 there is no x₂offset between echelon blocks, but in some designs it may beadvantageous to include such an offset. (Designs illustrating thesevariants will be described in sections 15 and 17, and illustrated inFIGS. 45 and 52.)

Multiple image fields are combined in field rows, e.g. row 4201 in FIG.42. The fields are exposed through separate projection systems and aresimultaneously scanned to cover an extended x₂ range in a singleline-scan operation. The fields are labeled as Field[ . . . ,i_(2 field)], i_(2 field)=0, 1, . . . (Field[ . . . , 0] corresponds tofield 4101 in FIG. 41.) All fields in a field row have the same x₃position, and have an x₂ offset (center spacing) of N_(dot/line)·G₂,where N_(dot/line) is the line-scan distance in dot units:N_(dot/line)=N_(dot/pulse)·N_(pulse/line), Eq. 38.5. If the line scanwere not terminated after N_(pulse/line) pulses, then the positionsoccupied by Field[ . . . , i_(2 field)] at exposure pulsesPulse[i_(line),N_(pulse/line)], Pulse[i_(line), N_(pulse/line)+1], etc.would coincide with the positions of the adjacent field, Field[ . . . ,i_(2 field)+1], at Pulse[i_(line),0], Pulse[i_(line),1], etc. Thus, thedot lines printed by adjacent fields seamlessly join to form extendedlines. (FIG. 42 schematically illustrates the exposed lines 4202 partway through the scan operation.)

Multiple field rows with different x₃ positions are simultaneouslyscanned, as illustrated in FIG. 43, to cover a printing surface such asthe semiconductor wafer 902 in FIG. 9. (FIG. 9 illustrates adistribution of projection lenses 901 above the wafer, whereas FIG. 43illustrates the corresponding image fields on the wafer surface. FIG. 27shows an alternative projection aperture layout.) The fields are labeledField[i_(3 field),i_(2 field)], where i_(3 field)=0, 1, . . . . (Thei_(3 field) index values correspond to field rows, with row 4201 in FIG.42 corresponding to i_(3 field)=0.) For notational simplicity, thelabeling includes some fields that do not exist. Only the fieldsillustrated as solid squares in FIG. 43 exist; those illustrated asdashed squares (e.g. Field[0,3]) are not used because they are near oroutside the boundary of the printing area 902. The field rows have an x₃offset (center spacing) of N_(line/field)·G₃, whereN_(line/field)=N_(line/row)·N_(row/field), Eq. 38.6. There is an x₂offset of N_(offset) dot units between field rows, which is indicated inFIG. 43 as −N_(offset)·G₂ (with the minus sign indicating thatN_(offset) is negative, as illustrated). In the FIG. 43 illustrationN_(offset) is the same for all field rows, but it may potentially be afunction of the field row index i_(3 field).

FIG. 44 illustrates the scan pattern traced by a particular field 4101relative to printing surface 902. All fields follow similar scan paths.(In practice, the printing surface is typically scanned relative tosubstantially stationary image fields.) Field 4101 traces multiple linescans, which are labeled Scan[i_(line)], i_(line)=0, 1, . . . ,N_(line/row)−1. Illumination pulse i_(pulse) in Scan[i_(line)] isPulse[i_(line),i_(pulse)]. The field first does a line scan (Scan[0]) inthe ê₂, direction from position 4401 to position 4402, makingN_(pulse/line) pulsed exposures (Pulse[0,0], . . . , Pulse[0,N_(pulse/line)−1]) while stepping by N_(dot/pulse)·G₂ between pulses. Itthen moves to position 4403, which is displaced from position 4402 byN_(row/field)·G₃ in the ê₃ direction. (No exposures are made in thisstep.) From position 4403 the field does a second line scan (Scan[1]) inthe reverse direction (−ê₂) to position 4404, making pulsed exposuresPulse[1, N_(pulse/line)−1], . . . , Pulse[1,0] (with the time sequencelabeled in decreasing order of the second Pulse index). The field thenagain steps by a distance N_(row/field)·G₃ in the ê₃ direction toposition 4405, and the process repeats until N_(line/row) line scanshave been performed.

Echelon block i_(block) in Field[i_(3 field),i_(2 field)] is labeled asBlock[i_(3 field), i_(2 field), i_(block)]; spot row i_(row) in thisblock is labeled Row[i_(3 field),i_(2 field),i_(block),i_(row)];focused-radiation spot i_(spot) in this spot row is labeledSpot[i_(3 field),i_(2 field),i_(block),i_(row),i_(spot)]; and the dotexposed by this spot in Pulse[i_(line),i_(pulse)] is labeledDot[i_(3 field),i_(2 field),i_(block),i_(row),i_(spot),i_(line),i_(pulse)].The x and y coordinates ofDot[i_(3 field),i_(2 field),i_(block),i_(row),i_(spot),i_(line),i_(pulse)]in the respective scan (ê₂) and cross-scan (ê₃) directions are given byEqs. 38.7 and 38.8.

15. Scan Configuration for Circular Image Field

The above-outlined scan configuration efficiently covers a rectangularimage field, but a variant configuration may be used to more efficientlycover a circular field, as illustrated in FIG. 45. The projection systemhas a circular design image field 4501 of radius R_(field), which ispartitioned into two subfields 4502 and 4503. Subfield 4503 has the samegeometry as subfield 4502, but rotated by 180° around the center ofcircle 4501. Each subfield comprises two juxtaposed rectangles of x₂dimension L_(block), where L_(block) is the nominal x₂ length of theechelon bocks 4001 in FIG. 40; see Eq. 38.9 in FIG. 38.

The two subfields scan separate sets of raster lines (indicateddiagrammatically as solid lines crossing the subfields), each set havingline spacing G₃. The two sets are interleaved to form a compositeexposure pattern with line spacing G₃/2 over the region 4504 where thesubfields' x₃ ranges overlap. The subapertures' upper and lower portions4505 and 4506 are outside of the overlap region, but they would overlapin different line scans to provide full coverage, at line pitch G₃/2,over most of the printing surface 902. (The position of subaperture 4502in a different line scan is indicated as 4502′ in FIG. 45.)

The aperture fill factor (i.e., ratio of composite subfield area tocircular field area) can be maximized by defining L_(block) according toEq. 38.10 in FIG. 38. Under this condition the fill factor would be77.6%. (By comparison, a circle-inscribed square field would have a fillfactor of 63.7%.)

16. Illumination Strobing

The interleaved raster line method would not work effectively, asdescribed above, with a continuous illumination source (such as theOXIDE laser, Ref. 23) because each focused-radiation spot only exposes asparse array of discrete dots with x₂ pitch N_(dot/pulse)·G₂ (FIGS.39A-39C). However, continuous illumination can be effectively strobed byeither using a beam-switching mechanism to cycle the illuminationthrough several projection systems, or by repeatedly scanning a narrowband of illumination across each microlens array at high speed. Thistechnique is described in the '986 patent (See the '986 patent's FIG. 25and associated discussion beginning at column 18, line 31.) The '986patent pertains to image-plane microlens arrays, but the strobe methodis equally applicable to object-space microlens arrays.

As illustrated in FIG. 46, a laser beam can be rapidly switched betweenalternative light paths by using a rotating element such as an opticaldisc 4601, which has multiple phase-Fresnel diffracting zones such aszone 4602 on its surface. An incident laser beam 4603 is focused ontoand transmits through (or reflects from) the diffractive surface, whichdeflects the beam into output beam 4604. The grating phase over eachzone is a linear function of the rotation angle, so that the phasegradient is angle-independent and the output beam direction will remainstationary as the zone scans the beam. Different zones deflect the beamalong different light paths, which are directed by downstream opticsinto different projection systems. (For example 19 zones could be usedto switch the illumination between the 19 projection lenses illustratedin FIG. 9.)

If N_(dot/pulse) is greater than the number of projection systems, thenthe scan distance during each illumination pulse would be greater thanthe printing dot pitch G₂ and significant “dot smearing” would resultfrom the raster scan motion. Under this condition—or if the laser poweris insufficient to supply all projection systems—each projection systemcan alternatively be equipped with a beam-scanning system similar tothat of FIG. 46, but configured to continuously scan a narrowillumination beam across the microlens array (in the manner illustratedin the '986 patent's FIG. 25). Other types of beam scanners, such aspolygonal-mirror, Risley-wedge, or acousto-optic scanners, mayalternatively be employed.

17. Microlens Aperture Geometries

FIGS. 47 to 52 illustrate several variant microlens aperture geometriesand array patterns in plan view. FIG. 47 illustrates a circularphase-Fresnel microlens 4701. The inner circles depict the Fresnel zoneboundaries.

FIG. 48 illustrates a phase-Fresnel microlens 4801 similar to element4701, but with its aperture truncated to a rectangular shape andnarrowed in the scan direction (ê₂). This aperture type would besuitable for implementing the pupil function represented by curve 2901in FIG. 29. Rectangular apertures can efficiently cover a microlensarray with very little or no fill-factor loss. FIG. 49 illustrates aportion of an array of microlens apertures including element 4801(shaded). The right portion of the array is illustrated with an echelonstep G₃ relative to the left portion. (The corresponding array ofimage-plane focal spots is illustrated schematically in FIG. 41.)

FIG. 50 illustrates a variant rectangular-aperture microlens 3401, whichis similar to element 4801 except that it has a half-cycle phasediscontinuity along its center line 3402 to create the pupil functionillustrated by plot 3101 in FIG. 31. FIG. 34 shows a perspective view ofthe lens.

FIG. 51 illustrates a microlens 5101 similar to element 3401, but withthe middle half of the aperture truncated to produce the dipole pupilfunction illustrated by plot 3601 in FIG. 36. The dipole-illuminationaperture form need not reduce fill-factor efficiency. FIG. 52illustrates a portion of a microlens array comprising dipole aperturessuch as aperture 5101 (shaded), which are interleaved to provide fullarea coverage with little or no fill-factor loss. (An echelon step G₃ isillustrated in the figure.)

18. 193i Design Example

FIGS. 53A and 53B tabulate illustrative design data for a spot-scanningimmersion lithography system using a wavelength (λ) of 193 nm, a waterimmersion fluid, and image-space numerical aperture (NA) of 1.35; Eqs.53.1 and 53.2 in FIG. 53A. (The high NA is possible because water has arefractive index of 1.437 at 193 nm.) The usable numerical aperture istruncated to approximate dimensions NA₂=0.604 in the scan direction andNA₃=1.207 in the cross-scan direction, according to Eq. 24.4 in FIG. 24with the additional condition that NA₃=2 NA₂; see Eq. 53.3. (Themicrolens aperture shapes, and the projection system's aperture stop,should both approximately match the 2:1 numerical aperture ratio inorder to maximize aperture packing efficiency.)

The system uses two sets of microlenses, which could possibly beinterleaved in the same microlens array, but are in separate exposuresystems for this example. The first microlens set is used to write nodallines as illustrated by diffraction plot 3201 in FIG. 32, and the secondset is used to trim the nodal lines as illustrated by diffraction plot3002 in FIG. 32. Microlenses in the first set have the form illustratedby FIGS. 50 and 34, with the pupil function represented by plot 3101 inFIG. 31. Those in the second set have the phase pattern illustrated byFIG. 48, and are apodized in the cross-scan direction as illustrated bypupil function 2902 in FIG. 31. Both sets of microlenses may similarlybe apodized in the scan direction.

The raster line pitch is G₃=λ/NA₃≅160 nm (equal to the nodal line pitch,cf. FIG. 32), and the dot pitch G₂ within each raster line is set to theoptical resolution limit λ/(2 NA₂)=160 nm (Eqs. 53.4 and 53.5; cf. FIGS.39B and 41). The microlens center spacings are at least 25 μm in thescan direction by 50 μm in the cross-scan direction, allowing for themicrolens aperture dimensions and any clearance space between themicrolenses. The projection system's reduction ratio is assumed to be25× (similar to the FIG. 1B design), so the x₂ offset between spots(N_(dot/spot)·G₂ in FIG. 39B) is at least 1 μm, and the x₃ offsetbetween echelon blocks (N_(row/block)·G₃ in FIG. 41) is at least 2 μm;Eqs. 53.6 and 53.7.

Eqs. 53.5 and 53.6 imply the limit N_(dot/spot)≧7 (Eq. 53.8).N_(dot/spot) is set to 8 (Eq. 53.9), implying a spot pitch in the scandirection of 1.279 μm (Eq. 53.10, FIG. 39B) and a microlens centerspacing in the scan direction of 32.0 μm (at 25× reduction). The 160-nmdot pitch will result in a fairly fast scan rate, but a slower,higher-resolution line scan can be performed by dividing G₂ by any powerof 2 and multiplying N_(dot/spot) by the same factor (leaving the spotspacing N_(dot/spot)·G₂ unchanged). The relative primality condition,Eq. 38.2 in FIG. 38, will be unaffected by this change becauseN_(dot/spot) will still have no prime factors other than 2.

Eqs. 53.4 and 53.7 imply the limit N_(row/block)≧13 (Eq. 53.11).N_(row/block) is set to 16 (i.e., 2N_(dot/spot), Eq. 53.12) so that themicrolens apertures have a 2:1 aspect ratio matching the numericalaperture ratio NA₃/NA₂. The cross-scan spot pitch is 2.557 μm (Eq.53.13, FIG. 41), and at 25× reduction the microlens cross-scan centerspacing is 63.9 μm.

The printing system comprises 38 microlens arrays and associatedprojection systems, with the apertures arranged as illustrated in theFIG. 27 plan view. The arrays cover most of a 300-mm wafer 902. Eachmicrolens array, such as array 2702, is rectangular with dimensions ofapproximately 25 mm in x₂ by 50 mm in x₃, and with a 5-mm x₂ clearanceand 10-mm x₃ clearance between arrays, as illustrated in FIG. 54. The25-by-50-mm array dimensions correspond to the projection systems'object field dimensions. (Each projection system's lens apertures andhousing must fit within the 30-mm by 60-mm footprint.) At 25× reductionthe image field dimensions are approximately 1 mm by 2 mm. Eqs. 53.14and 53.15 (cf. FIGS. 40 and 41).

Eq. 53.14 is combined with Eqs. 53.5 and 53.12 to obtain N_(dot/row)≈391(Eq. 53.16), and this is combined with Eqs. 38.3 (in FIG. 38) and 53.9to obtain N_(spot/row)≈49 (Eq. 53.17). N_(spot/row) is set to 49exactly, Eq. 53.18. (N_(spot/row) is equal to N_(dot/pulse), which mustbe an odd integer in order to satisfy the relative primality constraintof Eq. 38.2; cf. Eqs. 38.1 and 53.9.) N_(dot/row) is 392 (Eq. 53.19).The image field's x₂ dimension is 1.003 mm (Eq. 53.20), corresponding toan object field dimension of 25.1 mm at 25× reduction.

Eq. 53.15 is combined with Eqs. 53.4 and 53.12 to obtainN_(block/field)≈782 (Eq. 53.21). N_(block/field) is set to 784 (i.e.,2N_(dot/row)) in order to retain the field's 2:1 aspect ratio, Eq.53.22. The image field's x₃ dimension is 2.005 mm (Eq. 53.23),corresponding to an object field dimension of 50.1 mm.

The projection systems' center spacings are approximately 30 mm in x₂and 60 mm in x₃ (Eqs. 53.24 and 53.25 in FIG. 53B; cf. FIGS. 42, 43, and54). Eqs. 53.4 and 53.25 imply N_(line/field)≈375381, Eq. 53.26. FromEqs. 38.4, 53.12 and 53.21, N_(row/field)=12544 (Eq. 53.27), and Eqs.38.6, 53.26 and 53.27 imply that N_(line/row)≈30 (Eq. 53.28).N_(line/row) is set to 30 exactly, implying that N_(line/field)=376320and implying a 60.02-mm x₃ offset between projection systems (Eqs.53.29-31). N_(dot/line) is set to 188160 (i.e., N_(line/field)/2) toretain the 2:1 aspect ratio between the projection systems' x₂ and x₃center spacings, Eq. 53.32. The x₂ spacing is 30.01 mm, Eq. 53.33.

With a laser repetition rate (“rep_rate”) of 6 kHz (Eq. 53.34), the scanspeed is 47.0 mm/sec, Eq. 53.35. The number of fields N_(field) is 38(Eq. 53.36, FIG. 27), and the total number of focus spots N_(spot) inall fields is 23,356,928 (i.e. 614,656 per field), Eq. 53.37. Assumingthat the ON/OFF state of each spot is controlled by one data bit, thetotal data rate (“data_rate”) is 140 GHz (i.e. 3.7 GHz per field), Eq.53.38. The data rate would be higher if control capabilities such asgray-level control (discussed in section 19) are provided. The area scanrate (“area_rate”) is 35.8 cm²/sec (i.e., 94 mm²/sec per field), Eq.53.39. The total number of printed dots N_(dot) is 2.69·10¹² (i.e.,7.08·10¹⁰ per field), Eq. 53.40. The total print area coverage isapproximately 687 cm² (i.e., 18.1 cm² per field), Eq. 53.41. Thisamounts to 97% of a 300-mm wafer area, but includes off-wafer printingarea.

The scan time per wafer (“scan_time”) is 19.2 sec, Eq. 53.42. This doesnot include the time required for wafer loading and scan reversal, butis consistent with a throughput of order 100 wafers per hour. (However,two scans are required for the nodal-line and trim exposures, and thehigh throughput is also offset by the need for multiple patterning stepsto form dense line structures.) Assuming an exposure dose of 30 mJ/cm²(Eq. 53.43), the image-plane exposure power is 1.07 W, Eq. 53.44. Thelaser power would need to be higher to accommodate optical losses. Inaddition, much higher power (e.g. of order 10 W) may be required toexpose narrow lines using the low-threshold nodal-line printing methodillustrated in FIG. 32.

19. Supplemental Control Mechanisms

With a 6 kHz pulsed laser source, the optical modulators would switchthe focused-radiation spots on and off at the 6 kHz laser repetitionrate, which is very low compared to prior-art optical modulators.Furthermore, the modulators would only need to be latched and stableduring the very brief time interval of each laser pulse. The modulationcan be effected with micromechanical shutters proximate the microlensfoci, such as the modulator mechanism illustrated in FIGS. 19A-19D.

The relatively low modulation rate makes it possible to employsupplemental control mechanisms for individually controlling the spots'intensity levels (“gray level”) and center positions. These mechanismswould provide additional degrees of freedom that could be exploited forresolution enhancement, and they could also be used as correctivemechanisms. For example, gray level control can be used to vary linewidths and to compensate for variations in microlens transmittance andlaser power fluctuations. Centering controls allow the printed linepatterns to deviate somewhat from straightness and strict periodicity,and can also be used to correct small scan positioning errors and tocorrect thermally-induced image distortion.

A grating modulator such as that illustrated in FIGS. 23A and 23B canprovide both ON/OFF switching and gray level control. The modulator canprovide continuous gray-level control by positioning the movable elementat intermediate positions between the OFF and ON states. However, itwould be advantageous to use a grating modulator only for gray-levelcontrol over a limited transmittance range (e.g. 50% to 100%), whileusing a separate shutter mechanism for ON/OFF switching. For example,FIG. 55 illustrates a modulator 203 comprising a shutter mechanism 5501of the type illustrated in FIG. 19A in series with a grating modulator5502 similar to that illustrated in FIGS. 23A-23B but configured toprovide only gray-level control. (The grating modulator 5502 may beactuated by a comb drive mechanism, not shown.) In this mode of use, thegrating modulator need not be designed to the stringent tolerances thatwould be required to achieve a high extinction ratio in the OFF state.Also, the grating layers 2301 and 2303 in FIG. 23A can be thinner formodulating over a limited transmittance range.

The focused-radiation spot centering can be precisely varied over alimited range by equipping the microlenses with micro-mechanicalpositioning actuators. For example, if the projection system has a 25×reduction ratio, then a 25-nm translational movement of a microlens willinduce a 1-nm positional shift of the corresponding spot. A potentiallimitation of this method is that if the microlens is designed tocorrect strong optical aberrations in the projection optics, then thetranslational motion will induce additional optical aberrations. (Theinduced aberration is proportional to the gradient of the microlensgrating phase in the translation direction.) But this limitation can beovercome, as described below.

A centering control mechanism is illustrated schematically in FIG. 56,which depicts the spot-generation optics for a particularfocused-radiation spot. Two proximate microlens elements 5601 and 5602on the top of microlens/SLM plate 104 operate in conjunction to focusincident illumination through an intermediate focus 202 on the bottom ofthe plate. The intermediate focus 202 is at the object plane of aprojection system, and is modulated by a proximate modulator element 203(e.g., the shutter/grating mechanism of FIG. 55). Element 5601 isactuated to provide motion in the cross-scan direction (ê₃) for x₃ spotcentration control. (In this embodiment x₂ centration control is notneeded because the system operates primarily in line-printing mode.) Alow-power field-lens element 5603 on the bottom of plate 104 images themicrolens elements 5601 and 5602 substantially onto the system entrancepupil (which would be at infinity if the projection system istelecentric on the object side).

The FIG. 56 configuration is schematically similar to FIGS. 2, 3A and3B, except that the microlens element 201 in is split into two elements5601 and 5602, one of which is movable, and the field lens 5603 isadded. The field lens allows the aperture of element 5602 to operate asa pupil-defining aperture stop that is optimally customized for a singlefocused-radiation spot. The conjugate relation between theaberration-correcting microlenses and the entrance pupil enablescorrection of large optical aberrations, and it ensures that theimage-space focused beam remains centered in the exit pupil as element5601 moves.

The operation of elements 5601 and 5602 can be described as followsusing a thin-lens model, which is reliable when the two elements'phase-Fresnel structures are in close proximity. The phase-Fresnelsurfaces are modeled approximately as zero-thickness structures in acommon aperture plane. Coordinates in the microlens aperture plane aredenoted as x₂ and x₃ (corresponding to the scan and cross-scandirections, respectively). With element 5601 in its nominal centeredposition, the element's grating phase function is denoted as gp₁[x₂,x₃],and the grating phase of stationary element 5602 is similarly denoted asgp₂[x₂,x₃]. (Function arguments are delimited by square braces “[ . . .]”.)

When lens element 5601 is moved by positional increment x_(3 lens) inthe cross-scan direction, its grating phase becomesgp₁[x₂,x₃−x_(3 lens)]. The microlens motion induces a correspondingcross-scan positional shift in the focused spot's geometric image pointon the image plane (the printing surface). This positional shift isdenoted as x_(3 image), which is an implicit function of x_(3 lens). Atthe nominal centered position x_(3 lens) and x_(3 image) are both zero.

The projection system is characterized by an optical phase functionop_(proj.)[x₂, x₃, x_(3 image)] (also referred to as an “eikonalfunction”), which represents the optical path length from position(x₂,x₃) on the microlens aperture plane to the positionally shiftedgeometric image point. The illumination system is similarlycharacterized by an optical phase function op_(illum.)[x₂,x₃]representing the optical path length from the illumination source toaperture point (x₂,x₃). (The grating-phase and optical-phase functionsare defined in phase cycles.)

Perfect aberration compensation would be achieved if the totalsource-to-image optical path length, op_(illum.)[x₂,x₃]+gp₁[x₂,x₃−x_(3 lens)]+gp₂[x₂,x₃]+op_(proj.)[x₂, x₃, x_(3 image)], wereindependent of x₂ and x₃ for any x_(3 lens) within a design positionalrange. There are not generally enough design degrees of freedom toachieve this phase-matching condition exactly, but it can be achievedfor small x_(3 lens) values by using first-order differentialapproximations as outlined in FIG. 57.

The above-stated phase-matching condition is restated in Eq. 57.1, inwhich C is a constant in the sense of being independent of x₂ and x₃. Cmay be an implicit function of x_(3 lens). A first-order approximationis made to translate Eq. 57.1 to Eq. 57.2, with derivative terms definedin Eqs. 57.3-6: Dgp₁[x₂,x₃] is the derivative of gp₁[x₂,x₃] with respectto x₃; Dop_(proj.)[x₂,x₃,0] is the derivative ofop_(proj.)[x₂,x₃,x_(3 image)] with respect to x_(3 image) atx_(3 image)=0; M is the derivative of x_(3 image) with respect tox_(3 lens) at x_(3 lens)=0; and DC is the derivative of C with respectto x_(3 lens) at x_(3 lens)=0. Eq. 57.2 is required to hold for anyx_(3 lens), implying Eqs. 57.7 and 57.8. (C is evaluated at x_(3 lens)=0in Eq. 57.7.)

Eq. 57.8 is integrated with respect to x₃ to obtain gp₁[x₂,x₃], Eq.57.9. This is substituted in Eq. 57.7 to obtain gp₂[x₂,x₃], Eq. 57.10.The resulting design has several degrees of freedom, including theconstants C, DC, and M, and the function gp₁[x₂,0]. Substantiallywedge-free grating phase functions can be obtained by setting DC to zero(Eq. 57.11), and gp₁[0,0] can be set to zero (Eq. 57.12) because anyconstant phase offset can be absorbed in C. The choice of C isirrelevant to optical performance, but constant offsets can be appliedto gp₁[x₂,x₃] and gp₂[x₂,x₃] to adjust the positions of the Fresnel zoneboundaries. gp₁[x₂,0] can be set equal to gp₁[0,x₂] to make the gratingphase functions approximately axially symmetric, Eq. 57.13.

The remaining free design choice, M, can be selected to control thesensitivity of focus spot position to microlens displacement, or tobalance the optical power between elements 5601 and 5602 in FIG. 56. Ifall of the optical power is concentrated in the movable element 5601,then a lens displacement of x_(3 lens) will simply induce the samex_(3 lens) positional shift in the object-plane focus spot, resulting inan image-plane spot translation x_(3 image) approximately equal tox_(3 lens) times the projection system's magnification factor, i.e., theratio dx_(3 image)/dx_(3 lens) in Eq. 57.5 will be equal to themagnification. Thus, if M is set to the magnification factor, then mostof the optical power will be concentrated in element 5601, and element5602 will only function to preserve aberration compensation over thespot displacement range. (The magnification factor is the reciprocal ofthe projection system's reduction factor, and may be negative toaccommodate image inversion.)

It may be advantageous to set M to approximately half the magnificationfactor in order to balance the microlens optical power approximatelyevenly between elements 5601 and 5602. The spot positioning range wouldbe reduced by a factor of two, but positioning resolution would improveby a factor of two, relative to a design with M equal to themagnification factor.

The above design outline applies primarily to spot-generation opticsthat are designed to produce a pupil function such as that representedby plot 2901 in FIG. 29 (for a rectangular pupil). Based on thisstarting design, element 5602 in FIG. 56 can be modified to effect anapodized pupil function such as plot 2902 in FIGS. 29 and 31, or thephase-step pupil function represented by plot 3101 in FIG. 31 (cf. FIGS.34 and 50).

The shutter mechanism 5501 in FIG. 55 preferably comprises shutterapertures (1903 and 1904 in FIG. 19A) having the form of elongated slitsoriented in the cross-scan (ê₃) direction, which are MEMS-actuated tomove in the scan (ê₂) direction. This will allow the focused beam to bemoved in the ê₃ direction for centration control without being clippedby the shutter apertures. Also, the grating modulator 5502 preferablycomprises grating lines oriented in the ê₃ direction, with the movablegrating actuated to move in the ê₂ direction. The gratings'transmittance characteristics will be more uniform over the entrancepupil in this configuration because NA₂ is small in relation to NA₃, cf.FIG. 25. (A line grating's diffraction characteristics are generallycomparatively insensitive to ray direction variations in a planeparallel to the grating lines, relative to directional variations in atransverse plane, so it is advantageous to align the grating lines tothe wide aperture direction.) The grating lines can be elongated in theê₃ direction to accommodate beam centration control.

Any beam apodization in the ê₂ direction is preferably applied at theprojection system's aperture stop, not in the microlenses, because beamtruncation by the shutter apertures could otherwise interfere with theapodization. The shutter apertures' x₂ limits are preferably located atthe first diffraction nodes of the focused beam in order to minimizeboth the shutters' range of motion and the sensitivity to shutterposition in the ON state. The truncation of the focused beam'sdiffraction tails in the ê₂ direction will result in partial beamapodization at the image-space exit pupil. The optical attenuationprofile across the projection system's aperture stop can be tailored tofurther apodize the beam in the ê₂ direction.

Aberration control need only optimize the focus spot's cross-scan (x₃)resolution for line printing. Aberration-induced broadening in the scan(x₂) dimension can be tolerated, allowing the microlenses to be designedto generate resolved nodal lines at the shutter apertures' x₂ limitswhile also performing aberration compensation for line printing.

The Schupmann lens configuration 2101 of FIG. 21B can be modified toprovide beam centration control, as illustrated in FIG. 58, by makingthe top microlens element 201 movable. A small field lens 5603 proximatethe intermediate focus 202 images the entrance pupil onto element 201.The optical power can be balanced between elements 201 and 2102 tooptimize aberration compensation performance over the positional rangeof element 201 while maintaining substantial achromaticity.

20. Dual-Wavelength Lithography

The above-described nodal line printing method is applicable tomulti-patterning, single-wavelength lithography, in which multiplecoarse-pitch line structures are interleaved to form high-pitchstructures. The method is also adaptable to dual-wavelength processes,such as absorbance modulation optical lithography and two-colorlithography, which can perform the interleaving in a single exposureprocess without intermediate processing. Absorbance modulation isdescribed in U.S. patent application Ser. No. 13/103,874 (the '874application) and the references cited therein, and two-color lithographyis described in Ref. 25.

In this mode of operation, two focus spot patterns such as 3002 and 3201illustrated in FIG. 32 are simultaneously superimposed, using separateillumination wavelengths for the two patterns, to expose narrow lines ina photoresist. Pattern 3002 is illuminated with an “exposure wavelength”λ₁ which modifies the resist solubility in relation to the absorbedenergy. Pattern 3201 is illuminated with a separate “masking wavelength”λ₂, which inhibits the resist photo-activation by wavelength λ₁ so thatonly a narrow line of width w in FIG. 32 is formed where the λ₂intensity is below threshold t. Multiple lines can be scanned at a linepitch significantly smaller than λ₁/NA₃ by this method, without anyintermediate resist processing steps. (The line pitch G₃ in FIG. 41might be comparable to λ₁/NA₃, but multiple line scans can be used toexpose interleaved line patterns at a pitch much smaller than G₃.)

The '874 application disclosed methods for creating optical nulls atisolated points (as in the '874 application's FIGS. 26 and 27) by usingspiral-phase microlenses such as those illustrated in the '874application's FIGS. 18-23. Similar techniques are used in other priorart (e.g., as illustrated in FIG. 1 of Ref. 25). By contrast, theexposure pattern plot 3201 in FIG. 32 has an optical null along a line(3502 in FIG. 35), not at an isolated point. The use of line patterns,rather than isolated points, for lithography can greatly increase thesystem's achievable throughput and power efficiency.

In absorbance modulation optical lithography, a photochromic layer incontact with the resist operates as a contact mask, absorbing theexposure wavelength over regions where the masking wavelength has highintensity. For example, the cover plate 2012 in FIG. 20 could have aphotochromic layer on its bottom side, or a photochromic immersion fluidmight be used between the cover plate and resist. This type of processis described in the '874 application, which notes that it mayalternatively be possible to emulsify the photochromic medium in theresist. The latter method is analogous to two-color lithography, whichuses a photo-inhibitor in the resist.

The two wavelengths can be merged into the projection system by means ofa beam combiner, as illustrated schematically by element 112 in the '874application's FIG. 29. (A projection system similar to that illustratedin FIG. 1B could possibly accommodate a beam combiner in the spacebetween elements PL1 and PL2.) Alternatively, dual-wavelength optics canbe used throughout the optical system (e.g. from the source point 101 inFIG. 1A through the projection system), eliminating the need for beamcombining in the projection system.

Methods of achromatizing an optical system can be adapted to providedual-wavelength operation with two narrow-band sources. Phase-Fresneloptics (either transmitting or reflecting) can work with two widelyseparated wavelengths by using different diffraction orders for the twowavelengths. (For example, as noted previously, the phase-Fresnelstructure illustrated in FIG. 22C could operate simultaneously atwavelength 266 nm and 532 nm.) With orders m₁ and m₂ used for respectivewavelengths λ₁ and λ₂, a phase-Fresnel diffractive lens can exhibitsimultaneous blazing (high efficiency) and approximately equivalentraytrace properties at the two wavelengths if the approximate relationm₁λ₁≈m₂λ₂ holds.

A phase-Fresnel optic need not be perfectly corrected for chromaticaberration between λ₁ and λ₂. Some amount of chromatic aberration can beuseful for counterbalancing glass dispersion in the projection optics(e.g. by means of phase-Fresnel surfaces 108 and 109 in FIG. 1B). If thechromatic dispersion between λ₁ and λ₂ in the spot-generation optics issignificant, Schupmann-type microlens doublets (FIG. 21B) can be used tomitigate the dispersion. (The two Schupmann elements have sufficientdesign degrees of freedom to effect aberration correction at twowavelengths, bringing both wavelengths to a common focal point on theimage plane.)

Catoptric or catadioptric projection optics, such as the Schwarzschildmirror system of the '919 application or the catadioptric DUV system ofRef. 26, can be used for wideband or dual-wavelength operation.

A shutter-type optical modulator (FIGS. 19A-19D) can operate tosimultaneously block or transmit the two wavelengths, or it can operateto divert λ₁ radiation (the exposure wavelength) out of the opticalsystem via diffractive scattering (zero-order extinction) in the OFFstate. A diffractive optical modulator (FIGS. 23A and 23B) need only beoptimized for high zero-order extinction at wavelength λ₁ in the OFFstate, but it should be configured to have high zero-order transmissionof both wavelengths in the ON state.

The wavelength-λ₂ phase discontinuity illustrated by plot 3101 in FIG.31 can be effected by means of a phase-shifting optical surface such assurface 3301 in FIG. 33 or surface 3401 in FIG. 34 that induces a phasediscontinuity approximately equal to a whole number of phase cycles atwavelength λ₁ and a half-integer number of phase cycles at wavelengthλ₂. (For example, the phase step 3302 in FIG. 33 or 3402 in FIG. 34could induce a phase discontinuity of one cycle phase at λ₁ and ½ or 3/2cycle at λ₂.)

20. System Schematic

FIG. 59 schematically illustrates the components of a scanned-spot-arraylithography system and their functional relationships. Illuminatingradiation 5901 is directed onto a microlens array 5902 comprisingelements such as microlens 201, which focuses the radiation through anintermediate focal point 202 at the object surface of a projectionsystem 103. The projection system transmits the radiation to a printingsurface 105 at its image plane, and focuses the radiation from eachintermediate focus 202 onto a focused-radiation spot 702 on the printingsurface. An array 5903 of optical modulators proximate the intermediatefoci, including modulator element 203, modulates the radiationtransmitting to the focused-radiation spots. A mechanical stage 5904raster-scans the printing surface in synchronization with the modulationto record a synthesized, high-resolution optical image on the surface.

The synchronization is effected by a control mechanism 5905 (e.g., acomputer, digital micro-controllers, analog circuits, or a combinationof such elements). The control mechanism receives position information5906 from a position sensor or sensors 5907 (e.g., interferometricoptical encoders), which detect the positional relationship between theprojection system and the printing surface. The control mechanismgenerates positional control signals 5908 for the scanning stage andmodulation signals 5909 for the modulator array. The control mechanismmay also generate control signals 5910 for a mechanical actuator coupledto the microlens array, or for micromechanical actuators coupled toindividual microlens elements, which effect small positional changes inthe focused-radiation spots by moving the microlenses.

REFERENCES

The following additional patent references are referred to in thisdisclosure and are incorporated by reference:

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The following literature references and information sources are referredto in this disclosure. Literature references are incorporated byreference:

Ref. 1 Zemax is a commercial lens design program from Radiant Zemax LLC,based in Redmond, WA, USA (http://www.radiantzemax.com/). Ref. 2“TWINSCAN scanner evolution” in ASML Images, Fall Edition 2008, pp.14-15 (http://www.asml.com). Ref. 3 Coherent, Inc., Santa Clara, CA(http://www.coherent.com/). Ref. 4 “Review and Assessment of MeasuredValues of the Nonlinear Refractive-Index Coefficient of Fused Silica,”Appl. Opt. 37, 546-550 (1998). doi: 10.1364/AO.37.000546 Ref. 5 M. K.Yang et al., “Index of refraction of high-index lithographic immersionfluids and its variability,” J. Micro/Nanolith. MEMS MOEMS 8(2), 023005(2009). doi: 10.1117/1.3124189 Ref. 6 P. R. Dumas et al., “Applicationsand benefits of ‘perfectly bad’ optical surfaces,” Proc. SPIE 7102,Optical Fabrication, Testing, and Metrology III, 71020G (2008). doi:10.1117/12.797718 Ref. 7 K. Miyamoto, “The Phase Fresnel Lens,” J. Opt.Soc. Am. 51, 17-20 (1961). doi: 10.1364/JOSA.51.000017 Ref. 8 H.Rostalski et al., “Use of Diffractive Lenses in Lithographic ProjectionLenses,” in International Optical Design, Technical Digest (CD) (OpticalSociety of America, 2006), paper WD4. doi: 10.1364/IODC.2006.WD4 Ref. 9“Optical Properties of Thin Films for DUV and VUV Microlithography,” RITCenter for Nanolithography Research(http://www.rit.edu/kgcoe/microsystems/lithography/utilities.html). Ref.10 T. Nishimura et al., “Atomic scale characterization of HfO₂/Al₂O₃thin films grown on nitrided and oxidized Si substrates,” J. Appl. Phys.96, 6113 (2004). doi: 10.1063/1.1808245 Ref. 11 H. Hu et al., “Highperformance ALD HfO₂—Al₂O₃ laminate MIM capacitors for RF and mixedsignal IC applications,” Electron Devices Meeting, 2003. IEDM '03Technical Digest. IEEE International. doi: 10.1109/IEDM.2003.1269303Ref. 12 Foltyn et al., “Deposition of multilayer mirrors with arbitraryperiod thickness distributions,” Proc. SPIE 5193, Advances in MirrorTechnology for X-Ray, EUV Lithography, Laser, and Other Applications,124 (2004). doi: 10.1117/12.505401 Ref. 13 M. J. Vasile et al.,“Microfabrication by ion milling: The lathe technique,” J. Vac. Sci.Technol. B 12, 2388 (1994). doi: 10.1116/1.587769 Ref. 14 GD-Calc ®(Grating Diffraction Calculator),http://software.kjinnovation.com/GD-Calc.html. Ref. 15 Z. Bomzon et al.,“Radially and azimuthally polarized beams generated by space- variantdielectric subwavelength gratings,” Opt. Lett. 27, 285-287 (2002). doi:10.1364/OL.27.000285. Ref. 16 I. Richter et al., “Design considerationsof form birefringent microstractures,” Appl. Opt. 34, 2421-2429 (1995).doi: 10.1364/AO.34.002421 Ref. 17 D. C. Flanders, “Submicrometerperiodicity gratings as artificial anisotropic dielectrics,” Appl. Phys.Lett. 42(6), 492-494 (1983). doi: 10.1063/1.93979 Ref. 18 J. R.Karpinsky et al., “MEMS microshutter SLM for intensity modulation,”Proc. SPIE 3633, Diffractive and Holographic Technologies, Systems, andSpatial Light Modulators VI, 254 (1999). doi: 10.1117/12.349335 Ref. 19K. P. Larsen, Micro Electro Mechanical Devices for Controlling Light:Ph.D. Thesis, MIC - Department of Micro and Nanotechnology, TechnicalUniversity of Denmark, 2005. ISBN 8789935772 Ref. 20 J. H. Burnett etal., “High-index optical materials for 193 nm immersion lithography,”Proc. SPIE 6154, Optical Microlithography XIX, 615418 (2006). doi:10.1117/12.656901 Ref. 21 P. A. Zimmerman et al., “High index 193 nmimmersion lithography: the beginning or the end of the road,” Proc. SPIE7274, Optical Microlithography XXII, 727420 (2009). doi:10.1117/12.814381 Ref. 22 J. H. Burnett et al., “Birefringence issueswith uniaxial crystals as last lens elements for high-index immersionlithography,” Proc. SPIE 7274, Optical Microlithography XXII, 727421(2009). doi: 10.1117/12.814324 Ref. 23 OXIDE laser company,http://www.opt-oxide.com/en/. Ref. 24 International Technology Roadmapfor Semiconductors, 2011 Edition, Lithography,http://www.itrs.net/Links/2011itrs/2011Chapters/2011Lithography.pdf Ref.25 R. R. McLeod et al., “Two-color photo-initiation/inhibitionlithography,” Proc. SPIE 7591, Advanced Fabrication Technologies forMicro/Nano Optics and Photonics III, 759102 (Feb. 16, 2010). doi:10.1117/12.845850 Ref. 26 P. Huang and D. Leibfried, “Achromaticcatadioptric microscope objective in deep ultraviolet with long workingdistance,” Optical Science and Technology, the SPIE 49th Annual Meeting.International Society for Optics and Photonics (2004). doi:10.1117/12.559790

CONCLUSION

Scanned-spot-array imaging has multiple advantages for lithographyincluding maskless operation, modularity, relative simplicity and smalldimensional scale of the projection optics, low power requirement, lowscan speed, and accurate alignment and focus control. With the nodalline printing technique, such systems could provideultra-high-resolution and high-throughput printing capability inconjunction with multi-patterning or dual-wavelength recordingprocesses.

While the above is a complete description of specific embodiments of theinvention, the above description should not be taken as limiting thescope of the invention as defined by the claims.

1. A scanned-spot-array lithography system comprising an array ofmicrolenses and corresponding optical modulators, a projection system,and a scanning mechanism, wherein: the array of microlenses andcorresponding optical modulators, the projection system, and thescanning mechanism operate cooperatively to print a lithographic imageon a photosensitive layer when the layer is positioned proximate animage plane; each microlens receives radiation from a radiation sourceand focuses it into a convergent beam converging toward a correspondingintermediate focus; each convergent beam transmits through and divergesfrom the corresponding intermediate focus, transmits through theprojection system, and is focused by the projection system onto acorresponding focused-radiation spot on the image plane; the opticalmodulator corresponding to each microlens is positioned to intercept thecorresponding convergent beam proximate the intermediate focus, andoperates to modulate the radiation transmitting to the correspondingfocused-radiation spot; and the scanning mechanism raster-scans thephotosensitive layer relative to the focused-radiation spots insynchronization with the modulation to record a synthesized,high-resolution raster image on the photosensitive layer.
 2. Thescanned-spot-array lithography system of claim 1, and further comprisingcollimation optics, which receive divergent radiation from the radiationsource and direct it into substantially collimated radiationintercepting the microlens array.
 3. The scanned-spot-array lithographysystem of claim 1 wherein the microlenses are configured tosubstantially eliminate geometric point-imaging optical aberrations atthe focused-radiation spots.
 4. The scanned-spot-array lithographysystem of claim 1 wherein the microlenses are singlet microlenselements.
 5. The scanned-spot-array lithography system of claim 1wherein: the microlenses are Schupmann doublets, each doublet comprisingfirst and second microlens elements; and the first microlens element ofeach doublet focuses radiation toward the corresponding intermediatefocus, the second element receives radiation diverging from theintermediate focus and further diverges it; and the first and secondelements are configured to substantially eliminate chromatic aberrationat the corresponding focused-radiation spot.
 6. The scanned-spot-arraylithography system of claim 1 wherein the microlenses comprisephase-Fresnel elements.
 7. The scanned-spot-array lithography system ofclaim 1 wherein the projection system comprises at least onephase-Fresnel lens surface.
 8. The scanned-spot-array lithography systemof claim 1 wherein the radiation source is monochromatic, themicrolenses and the projection system are configured to producesubstantially zero-intensity nodal lines at some or all of thefocused-radiation spots, and the scanning mechanism raster-scans thephotosensitive layer in the direction of the nodal lines.
 9. A method ofprinting a synthesized, high-resolution raster image on a photosensitivelayer proximate an image plane by exposing the photosensitive layer to anodal line exposure pattern and a trim exposure pattern, wherein: ascanned-spot-array lithography system of claim 8 performs the nodal lineexposure; and selected portions of the nodal line pattern are exposed bythe trim exposure.
 10. The printing method of claim 9, wherein the trimexposure is performed by a scanned-spot-array lithography system thatcomprises an array of microlenses and corresponding optical modulators,a projection system, and a scanning mechanism, wherein: the array ofmicrolenses and corresponding optical modulators, the projection system,and the scanning mechanism operate cooperatively to print a lithographicimage on a photosensitive layer when the layer is positioned proximatean image plane; each microlens receives radiation from a radiationsource and focuses it into a convergent beam converging toward acorresponding intermediate focus; each convergent beam transmits throughand diverges from the corresponding intermediate focus, transmitsthrough the projection system, and is focused by the projection systemonto a corresponding focused-radiation spot on the image plane; theoptical modulator corresponding to each microlens is positioned tointercept the corresponding convergent beam proximate the intermediatefocus, and operates to modulate the radiation transmitting to thecorresponding focused-radiation spot; and the scanning mechanismraster-scans the photosensitive layer relative to the focused-radiationspots in synchronization with the modulation to record a synthesized,high-resolution raster image on the photosensitive layer.
 11. Thescanned-spot-array lithography system of claim 1, wherein: the radiationfrom the radiation source comprises first and second distinctwavelengths; the microlenses and the projection system are configured toproduce intensity maxima in the first wavelength coinciding withsubstantially zero-intensity nodal lines in the second wavelength atsome or all of the focused-radiation spots; and the scanning mechanismraster-scans the photosensitive layer in the direction of the nodallines.
 12. A method of printing a synthesized, high-resolution rasterimage on a photosensitive layer proximate an image plane by exposing thephotosensitive layer to focused-radiation spots comprising intensitymaxima at a first wavelength coinciding with nodal lines at a secondwavelength, wherein: a scanned-spot-array lithography system of claim 11performs the dual-wavelength exposure; and wherein the second wavelengthinhibits photo-activation of the photosensitive layer by the firstwavelength.
 13. A scanned-spot-array system comprising multiplesubsystems of claim 1 configured to operate in parallel and tosimultaneously print onto a photosensitive layer on a common imageplane, wherein the separate subsystems comprise separate microlensarrays, modulators, and projection systems.
 14. The scanned-spot-arraylithography system of claim 1, wherein each modulator comprises amicromechanical shutter mechanism.
 15. The scanned-spot-arraylithography system of claim 1, wherein each modulator comprises twoproximate transmission diffraction gratings, one of which is actuated tovary the convergent beam's zero-order transmittance through bothgratings between a substantially zero-transmittance OFF state and ahigh-transmittance ON state.
 16. The scanned-spot-array lithographysystem of claim 1, wherein each modulator comprises: a micromechanicalshutter mechanism for effecting ON/OFF switching; and two proximatetransmission diffraction gratings, one of which is actuated to effectgray-level control by continuously varying the convergent beam'szero-order transmittance through both gratings between low, high, andintermediate transmittance levels.
 17. The scanned-spot-arraylithography system of claim 1, wherein each convergent beam traversestwo microlens elements, one of which is micromechanically actuated toprovide spot centration control.
 18. The scanned-spot-array lithographysystem of claim 17, wherein the microlenses are configured tosubstantially eliminate geometric point-imaging optical aberrations atthe focused-radiation spots and to maintain substantial elimination ofaberrations over the full actuation range of the centration control. 19.A method of printing a synthesized, high-resolution raster image on aphotosensitive layer proximate an image plane, the method comprising:directing radiation from a radiation source through an array ofmicrolenses and corresponding optical modulators, through a projectionsystem, and onto the image plane, wherein: each microlens receivesradiation from the radiation source and focuses it into a convergentbeam converging toward a corresponding intermediate focus, eachconvergent beam transmits through and diverges from the correspondingintermediate focus, transmits through the projection system, and isfocused by the projection system onto a corresponding focused-radiationspot on the image plane, and the optical modulator corresponding to eachmicrolens is positioned to intercept the corresponding convergent beamproximate the intermediate focus, and operates to modulate the radiationtransmitting to the corresponding focused-radiation spot; and operatinga scanning mechanism to raster-scan the photosensitive layer relative tothe focused-radiation spots in synchronization with the modulation torecord the synthesized, high-resolution raster image on thephotosensitive layer.